The juniors will have to read documentation and spend long hours writing and reading code and understanding what has already been built. Just like all of us who did that and had to do it in order to move up. The idea of spending time "mentoring juniors" has always ended up being an euphemism. It's just a way for non-technical people, at the end of the day, to consume as much time as possible from other people so they can pretend to be doing work.

It's clear which juniors are motivated and will make the most of the mentorship and which ones are not. Some are just there for the check and you cannot compromise your senior and higher talent on people who don't want to learn and grow and just want the prestige and check of a high-tech job. It's the same situation with someone who is drowning not listening to orders and taking down their rescuer with them too.

If a junior needs someone to sit next to them because they toil for way longer than needed on clear simple requirements and goals, this may not be the field for them. I never needed anyone right next to me that couldn't have been a direct message or a brief remote call to clarify something specific, clear, and to the point. You're describing a reality where the people who don't do are just trying to take as much time for themselves as possible to hide the fact that they don't want to think.