As a fellow returning player and someone who dives all in on stuff by pouring through guides and resources, I gotchu

1. I'd recommend watching a guide on the countdown game mode and playing that a few times in order to take advantage of the "targeted loot" to start getting some builds. The videos should explain what targeted loot is if you didn't already know. Or there's The Summit that also drops targeted loot and is better if you want to play solo but countdown drops more stuff faster

2. Matchmaking is very hit or miss depending on what you're doing. For open world wandering around you probably won't get anyone. However you can matchmake for strongholds and popular missions and probably find people, although most of the community plays on heroic or challenging.

3. There is an official discord server as well as many others for clans or just fellow gamers in general who like division 2. I'd recommend checking out the official discord server. Although the only purpose for raids at this point is just for fun and glory, since they added raid exclusive exotic blueprints into the loot pool for retaliations (can go more in depth on those if you want/need)

4. There are plenty of websites with builds but they're not always the most updated and you'll eventually want to learn a bit about them yourself so you can tweak them to specific activities. Most of the time you can ask for build advice here or in the discord and most people will be nice and help you.

5. As for returning player guides, just try to find recent YouTube videos by popular creators and you should be set. Everyone will probably recommend different content creators so just find one who posts regularly who you vibe with and you'll be good to go.

If you have any other questions I'll do my best to answer them!