I'm trying to do some development of some custom upfront themes so I'm taking builder for a walk.
When I click "Edit in Builder" I get redirected to a subdomain off my main domain which is matches the "theme directory" value. This is true whether I create a new theme or try to edit an existing one.
My first attempt was using a local dev env using local by flywheel (local.getflywheel.com) but I also setup a public site for this (themes.littlefyr.com). I've tried this using both the Brave and Chrome browsers with the same effect.