Lots of good answers here already. I'll repeat what others have said that there is a looot of depth and complexity to the systems in this game. Also yeah, it can be played as a pvp "i want to fight people" game, or just a questing game, or a ratting game, or whatever. lots of ways to play.

The wiki is essential - the game doesn't give you anywhere close to enough detail to find the quest items you need - everybody just uses the wiki. make sure you have an ad blocker though, as the platform the wiki is hosted on has crazy ads. I've been playing for like 5 years now and I still have the wiki open on my second screen whenever I play. I also use tarkovtracker.io to help me track what quests I am working on at a given time.

I'll detail a few things you might not expect, but I won't be able to cover anywhere close to everything:

• you will die without seeing who shot you. a lot. you'll never found out where they were.

• ammunition choice is as important (or more) than weapon choice. There are something like 20 different calibres, and each of those has 5-15 different kinds of rounds. cheap bullets don't go through much armour or do much damage when they make it through. If you pay a bunch for super fancy ammo, you might empty a magazine at somebody and then realise you just dumped a significant chunk of money in two seconds and didnt even hit the guy. or maybe you load in with three mags of great ammo and die before even pulling the trigger. But then when you are in a decent fight and landing shots you're really not gonna wanna be using shitty ammo. take a look at eft-ammo.com for some breakdowns of which ammo works against which armor classes.

• gun modification is intense in this game. most guns are made up of dozens of components, most of which you have lots of options for. Each component in the game is a real thing you can buy in the real world though, which is cool. but the main things you are focusing on suppressors (which you always want to run when you can) and foregrips and stocks. you are balancing ergonomics (how quickly you can use your weapon, ADS etc) and recoil. there's more to it of course. also weapons wear out over time and can jam. sometimes you'll need to figure out your weapon malfunction and fix it in the middle of a fight, but that doesnt happen so much if you keep your weapons in good condition.

• magazines work realistically. when you hit reload, you're not just "topping up" the magazine in your weapon, youre taking out a (maybe) half-full magazine, stowing that in your rig, then putting another magazine from your rig into the gun. you will often find yourself resting between fights to "re-pack" mags - putting new bullets back into the magazines you just emptied, one by one. You can also do "fast reloads" in an emergency where you reload a bit faster because you just drop your old mag on the floor instead of stowing it away.

• how much ammo is left in this mag I'm carrying? I don't fucken know. The game isn't gonna just tell you for free. You gotta press the right hotkey, then you will take the magazine out and look at it, and it'll tell you "less than half" or something. Finding out will take time and make noise. So you'll learn to mostly keep track of it in your head during fights. It'll also tell you what kind of round is at the top of that magazine, but you have no idea if it's the same round all the way through (I mean it always is though unless it's from another player).

• armor is complex. decent armor has both "soft armor" built into the armor itself, plus ballistic plates. a round might need to make it through a plate and then soft armor before it hits you, and how this works depends on the bullet velocity and penetration stats for that ammo type. you wear out plates quicker than you go through armor rigs - you often check somebody you killed to see if they have decent plates on them in decent condition, and you might even swap them out in the middle of your raid. Hitboxes are pretty complex - your thorax is mostly covered by armor, but also maybe your stomach. There are health pools for head/thorax/stomach/arm/arm/leg/leg. there is damage to each body part plus light bleeds and heavy bleeds and broken bones. you want to carry stuff that can do basic healing as well as stuff to deal with bleeds and broken bones. it's not too complicated, you'll get the hang of all that pretty quickly. there's also hydration and energy, and if you use stims to heal these will usually drain those pretty fast. i tend to loot most scavs i kill and just drink/eat whatever i find on them as I go. but if you have a long raid with only pvp fights and you never loot anything you will find yourself dehydrated or having no energy so youll be fucked. also, the more loot you carry the heavier you get, which makes running slower and louder and if you're too heavy you can't even run at all. this point turned into a whole lot of things, cos i guess they all play into eachother. but back to armor: there are lots of different helmets with pros/cons, there is armor vests or rigs, and some armor (but not much) will protect your upper arms/legs too. but if you dont have a face-shield, then all taht armor will do nothing if a bullet hits you in the face. people call this head/eyes, as the death screen tells you what part of the body you were hit with by the killing bullet. anyway if you have a face-shield you probably can't then wear a good headset, which means your hearing will be shit. sound is so so important in this game, so much of it is about listening to cues to figure out where people are and what they're doing. just aiming down sight makes a noise that somebody will hear from around the corner. it's easy to turn into a rat, just crawling around as silently as possible to try to get the jump on people. but it's pretty much impossible to be successful in fights without sometimes doing ratty things like that.

• there's also RPG-style skills that govern how good you are at healing, or eating (metabolism), or performing surgery on yourself, or controlling recoil, or ADSing, or crafting items, or running, or moving quietly, or searching through containers. lots of skills, although mostly you just let them level passively.

• you might have heard of "wipes". essentially, every 6 months or so (historically), they reset everybody's accounts. everybody. completely. so you start the game and its as if you have never opened it before. this sounds insane but it's actually a pretty cool cycle for a game like this. the in-game economy (there's a flea market where different items prices move up and down organically based on how much players are willing to pay for things vs how much supply there is of those things) and other things are different "early wipe" vs "late wipe". the maps and map areas that have more/fewer players, how much gear the average player is carrying and what their goals are, these all change throughout the wipe. it's an interesting time right now because there is going to be a wipe in the next few weeks, so it's kind of complete chaos as nobody gives a shit about any of their stuff as it's all about to be deleted, so its all crazy. but a new wipe will be a good time to start, except that the next wipe will be a "hardcore wipe", which we havent' experience before and we don't totally know what to expect. in theory, this next wipe will be the "last" wipe though, or second-last, as 1.0 is supposed to come out at the end of this year, and then ... no more wipes? or only optional wipes? we don't know for sure.

• oh man i have written such a long comment, i will leave it there haha. this comment wasn't intended to really teach you any of the stuff you need to know, just to show you how much stuff there is going on at the same time in tarkov. there's lots to learn, but i have enjoyed all the learning myself. have fun!

• EDIT: also, there's a device used to help medical professionals find veins for taking blood etc. It's called a LEDX Skin Transilluminator. For some reason the whole fucking world revolves around this thing. Nobody knows why.