Like with any kind of learning, without a feedback loop (as tight as possible IMHO), it's not gonna happen. And there is always some kind of feedback loop.

Ultra short cycle: Pairing with a senior, solid manual and automated testing during development.

Reasonably short cycle: Code review by a senior within hours and for small subsets of the work ideally, QA testing by a seperate person within hours.

Borderline too long cycle: Code review of larger chunks of code by a senior with days of delay, QA testing by a seperate person days or weeks after implementation.

Terminally long feedback cycle: Critical bug in production, data loss, negative career consequences.

I'm confident that juniors will still learn, eventually. Seniors can help them learn a whole lot faster though, if both sides want that, and if the organisation lets them. And yeah, that's even more the case than in the pre LLM world.

LLM can also help learning f you ask it what can be done better. Seniors can make prepromt so that company customs are taken into account.