...Yet, when ich try MP out, I often get insulted for beeing a bad player.
I have played about 50 MP matches so...
For a game that has been out for so many year you are "noob", but that is not a bad thing and it certainly does not give others the right to insult you.
I would suggest thou that you start by playing 1vs1 where you can learn faster. Then move to 4vs4 where a weak player has less impact in the game.
I would also suggest that you watch the replay to see how you opponent won. Then you can try the same tactic and see your opponent will counter it, and so on...
-How do the "Osttruppen" acutally work?
Ost work best in cover or garrison because they get a bonus accuracy.
(for a weird reason is they down to 2-3 and have picked weapon they have more damage out of cover because picked up weapon actually suffer a penalty when in cover.
-What a good commanders to pick?
Depends on you play style and faction for Ostheer the ones with g43s are considered good.
-Which "bulletins" affect exactly which unit?
should be accurate for most units
https://coh2db.com/bulletins/index
-Do get brit inf bonus while in cover?
all infantry get bonus. ?In addition IS get penalty when out of cover, while Ro.E get bonus when in cover.
-Does it matter which infantery type captures a weapon?
re-crewed weapon have the base stat of infantry that crew the weapon. The also reinforce at that cost and have that pop.
Some give can be found here:
https://community.companyofheroes.com/categories/guides