deuxhero
Arcane
It is through in-game dialog. Mage's Guild tells you the questline of that area is for new recruits. There's plenty of high level stuff in Old Ebonheart release though, like
There's also alternate start mods (Chargen Revamped - Expanded Lands for example, but that hasn't updated and I expect trying to start in Firewatch would go poorly), so you can skip traveling there normally and just start there.
Lich, Daedric realm megadungeon
There's also alternate start mods (Chargen Revamped - Expanded Lands for example, but that hasn't updated and I expect trying to start in Firewatch would go poorly), so you can skip traveling there normally and just start there.