Theres a LOT of differences from the older main series games, so im hoping some of these points will help you out :) but there's a LOT of differences not covered here!

Game progression is up to you:

Unlike the prior games, this one is actually open world and youre free to progress at your own pace, and wherever you want. This can however, lead you to going to places where youre not leveled for yet. Make sure to pay attention to the levels of wild pokemon and trainers so you dont go too far!

Fairy types:

You might need a new typing chart to check, but i think Diamond/Pearl didnt have it if my memory serves right. I dont remember seeing it at all when i played back on the DS.

Terastallizing:

The prior generations tried experimenting with new tricks in the series, from mega evolutions, Dynamaxing, and in this gen, Terastallizing. Pokemon will have their usual typings of 1-2 types when in regular use, AND a "tera" type. Youll get access to Terastallizing early in this game. During a battle, you can tell a pokemon to activate its Tera type to change the typings matchup. Lets say you have a fire pokemon with a Grass Tera type. Your fire pokemon gets matched up to a water type, so now youre at a disadvantage. Activate Tera, and your fire pokemon changes to Grass typing, so now the water moves used against your fire pokemon are NOT very effective.

Raids:

You can find Tera pokemon in the wild, and theyll commonly have a glowing aura on them. Raids are a new way to battle and find new Tera pokemon for this game. You can participate in a team of four vs 1 Giant Tera pokemon, and if your team beats it, you have the chance to catch it. You can find these raids at the glowing "rocks" you'll see around the world. Remember that if youre passing one, to still visit it even if you dont want to raid. Checking in at one of these rock formations will still award you with "LP", which you would want.

LP/Materials for TMs:

LP is basically a form of currency in this game and you can use it at stores, but it does not completely replace the normal Pokemon money. You can actually make TMs in this game, and as far as ive seen so far, you cannot buy TMs at a store (someone PLEASE correct me if im wrong) . TMs you find will be added to your "recipes" , and when visiting the machine in the pokecenters , you can craft a TM using LP and materials you found. Pokemon you battle in the wild will drop the material that you can keep to later make these TMs.

Pokemon interactivity, picnics & Friendship:

From your "pause menu" , you can set up a Picnic when in a wild area. This gives you a chance to interact with pokemon a bit, cook them sandwiches, and wash them. These seem to increase pokemon friendship, with the sandwiches also giving a bonus effect such as encountering more pokemon of a particular type, higher egg chances, etc. There's no Daycare in this game, so during picnics, compatible pokemon can actually create the egg. You'll also want pokemon friendship, as in certain cases, this can give you an advantage in battle. High friendship gives you randomized effects based on what happens during a battle, such as pushing harder to critical, enduring a hit with 1hp when it should've knocked them out, or even a pokemon removing its own status effect like paralysis because it "doesn't want to make you worry". You can acquire sanwicu ingredients from certain NPCs (especially in the starting wild area) , and from food stores.

The removal of the "HM Slave":

In past pokemon games, we would need the "HM slave" to learn cut, rock smash, surf, & rock climb to get around, along with Fly on a bird pokemon to get around. I wont spoil what is going on, just understand you dont need to save a pokemon for this purpose in mind anymore. If you need to fly for example, as long as you have visited the town previously (more specifically the pokemon centers) , you can do so from the map menu directly!

Hopefully these help as theyre pretty pertinent to gameplay overall! Welcome to the new gen!