From where I sit, "remote first" is all too often a code word for "only senior devs need apply".
You give them an offer.
>How do you mentor them and (over years) help them grow into a senior dev?
Same way you do in an office but over video chat.
When we bring on a junior, it's typically targeting a specific effort, with a specific team/mentor structure, and a desired exit strategy - at the end of X months, we expect the person will be capable of Y and Z, at which point they can fill these rolls in this other planned effort.
The mainline developer track isn't too tough to negotiate (a Java Dev 1 can do X, a Java Dev 2 can do Y, and so forth), but the harder part of the journey is cultivating breadth. Getting them experience with setting up a build system, or alternate OS familiarity, or learning color theory, or how to interpret a security scan.
Are there any "remote first" bootcamps that don't require students to be in the same timezone?