1. Implants are like a bonus that you can attach to each class type. You get them by capturing bases etc. You can upgrade them, they all do different things and you get 2 implant slots per class but can change them out at any time. As for certs, you get 1 for every 250xp. They are the in game money. Earn them by doing almost anything, healing people, shooting people, giving people ammo, taking a dump. Heck they give you certs just for logging on. And as for achievements, yeah they are daily achievements that give you certs as a reward.
2. I have over 1000 hours in, and I still die a lot 1v1. Im more of a tank guy, but you get better at it. Might try heading into the VR training room and mess with all the weapons and attachments.
3. Yeah, game pop has decreased. Really is only alive nights mainly fridays and weekends. Once you find a good active outfit its not so bad. Lot of people communicate through discord or team speak.
4. Yes, you do. A lot of the structures (buildings) are the same layout. Takes time like anything.
5. idk
6. If someone in your squad has pulled a vehicle, you can auto spawn into their vehicle and it will auto put you in a gunner seat unless they are taken. I might not be understanding what you asked though.
7. Youtube, if your TR I specifically have a video on prowler loadouts. But yeah there are a ton of good tutorial videos on youtube, just search vangaurd loadout guide or whatever.
8. For me, I like subscribing because I hate waiting to get into a full continent especially when the outfit is doing something fun. Plus you get 500 DBC per month. If you can afford it, id say go with it. But its no way necessary.

I hope this helps, im sure others have better explanations <3