Yggdrasil collections
You can be in any class.
You can be any class. If you have the collections unlocked for several items it supposedly prioritizes the one you’re currently playing, so there might be an argument for going ranger for that part.
I can confirm both of the things, above.
You can get it on any character, BUT, if you have other collections open it may go to one of those first. If you 100% want it on a particular specialization FIRST, kill the mushroom on that class.