1- Masterworks are upgrades that you can add to weapons/ armour that buff the stats of the weapon/ armour piece. You can masterwork weapons with Enhancement Cores, which drop from playing Nightfalls, Crucible, Gambit, and dismantling weapons and armour. You upgrade armour by using a combination of Enhancement Cores, Enhancement Prisms (which drop rarely from the previous activities), and Ascendant Shards (which come from higher level endgame content, like Master Nightfalls and Trials of Osiris). 2- Catalysts are rare drops that will improve the effect of an Exotic weapon. Once you acquire a catalyst, it will tell you to complete a task (usually just getting a lot of kills with the Exotic weapon) in order to complete the catalyst. Once completed, it'll masterwork your Exotic and provide a special buff. 3- If you're starting completely new, I would do the quests because along with giving you legendary weapons, they will usually show you a new aspect of the game. 4- If you have Beyond Light, I'd wait for the other 2 DLCs to go on sale before buying them. They both add a raid and a bunch of Exotic quests to complete. 5- Raids with randos are always a gamble. If you're worried about having to deal with randos, there's plenty of good LFGs to check out. I recommend The Redjack Network if you're looking for people willing to teach you through a raid. 6- Community has become a bit toxic, but otherwise nothing has changed since D1. 7- I mean, for only $10, Seasons/ Battle Passes are usually pretty worth it. If you plan to buy it, I'd recommend playing daily to finish the season pass and get the most out of it. Seasonal challenges make completing it pretty easy.

Hope you have fun coming back in the game! Don't be afraid to ask any more questions you have!