All will show you stuff from every community from every Lemmy instance, unlike local which only shows you communities from your local instance.
Not exactly all instances, but all instances federated with and cached by your home instance. (Sometimes it doesn't sync quite right)
Indeed. OP, you are on lemmy.ml, one of the largest instances, so you'll see almost all the content there is on Lemmy.
Someone on a much smaller instance wouldn't see that much.
Also, welcome!
I think that's what shocked me. I went into all and couldn't figure out rhyme or reason I was seeing so much!
Yes that's correct.
All has posts from all the communities the members on that instance subscribe to.
Support / questions about Lemmy.