Congrats on your new pup! I'd strongly recommend finding a good local dog club and enrolling in basic obedience. It's great you're watching videos and online can be very helpful, but I don't think there's any substitute for classes. As for the biting, it doesn't sound like an aggression thing from what you're describing-just that she's a GSD pup who's likely teething, and hasn't learned bite inhibition yet. Biting when getting treats, is it TAKING the treats that she "bites"? I had to wear bandaids on my thumbs for awhile training my pup as he learned to "take gentle" because his shark teeth would catch on the folds of my thumb skin :ROFL:
Keep the treats closed in your fist and don't open it until she gently licks or noses your hand-THEN she can get her goody. Hand feeding parts of meals can help with this too.
Saying "No" multiple times will achieve nil because to her the word doesn't mean anything, likely because it's repeated, she's tuning it out. Give a command or a word ONCE, and make sure she knows what it means (tone of voice doesn't necessarily override the reward aka pleasure shes getting out of nipping.) Redirect the biting to something appropriate, and remember that puppies are drawn to movement. Moving hands, moving legs...all that is very tempting. If she get's in crazy shark mode, and redirection isn't doing anything, try a high yip, getting up and walking away, or putting her in her kennel to calm down (I don't get angry or yell, I just firmly say "nope" and separate for a short bit.) They learn that biting means fun stops. Being gentle means we can keep having fun.
Again, this isn't unusual behavior. It's not acceptable behavior, obviously, but no need to panic
In terms of fetch, shepherds aren't retrievers, so don't expect this to necessarily come naturally, especially at such a young age. She probably doesn't know that you want her to bring it back, and she likes having it, so why should she drop it? All this can be taught with patience and steady fun training. We have to teach them what we want before we can expect it from them. It will take time but it's so rewarding!
Enjoy her-it sounds like you've got a fun little fireball!