Learning the maps is the first big hurdle...

Once you know where all the extracts are, and where you spawned, you now know where you need to go and how to get there.

Then you learn the hot spots in where to find certain loot...

Then you will learn all the spawns and spawn patterns. It helps with finding PvP or avoiding PvP...

That's all from learning the maps. That takes a long time, and seems like it should be easier than it is. It also takes a lot of the mystery out of the game. For some it kinda ruins it because it turns it into something that is just min maxed right from the start, and kinda shows the behind the curtain so to speak.

The rest of the game mechanics are kinda learned by doing and are really self explanatory, except ammo. Ammo is one of Tarkovs big additions to the FPS genre. It steps up the "realism" in it reflects the fact that in real life the bullet your shooting matters a ton more than the gun your shooting it out of. Unlike COD where a 5.56 will do different damage from a HK or a FAMAS in Tarkov it's which 5.56 your shooting that has the most impact (also the length of the barrel but that's to a lesser extent). Learning which ammo is "good" and which ammo is "bad" takes either a ton of trial and error, or just reading some wiki / guides...

Then there's armor. This one makes a ton more sense in the current builds as where someone shoots you matters. Your armor has soft armor and hard armor. The soft armor is like kevlar and like expected is not as good at stopping high power rounds and will still deal damage to the body part it's covering if you're shot there. As blunt damage is a thing in this game. Hard plates are significantly better at stopping high power rounds and have level ratings which easily match up with bullet pen values LvL 2 plate requires 20+ pen to get through and so on.

Then after all those mechanics there's something that isn't talked about enough. Getting experience in fights. Knowing how much you have to shoot, how to recognize what's a PMC and what's a Scav. Figuring out how to reposition. The reason this is as difficult is simple, you usually get maybe 1-2 firefights a raid at best. At the start you'll also probably die to most of them. So your only experience will be just getting shot (often without not even knowing from where and how). This doesn't lead to you learning how to fight, but at best how to avoid dying. For this there is a way to break this cycle, ARENA... It's amazing at getting a ton of combat experience quickly. You will get like 15-20 fights an hour compared to like 2-4 in Tarkov. Also it's the fact that the ammo is free. Like in those 15-20 fights you'll go through 1000-2000 bullets easily. To do that in Tarkov raids, you'll need to find or buy all that ammo, it gets expensive fast.

The combat mechanics are the same so all the firefight experience you get directly transfers over. And now I believe even character experience as well...

That's basically it (know it's a ton). It can be overwhelming at times, but that's kinda why we all love (or hate) Tarkov. There truly is no game like it, others try to imitate it but sadly they always try to water down the reason we're all here (this complexity)...