That doesn't sound like a WFH problem

We jad a junior to train while we all WFH, but he was a very receptive and smart guy, so it was kind of easy. The last stage of code review, we would go through all changes together in a meeting. I would explain the reasons something may be a bad idea (e.g. harder to maintain later on). Other things were also handled in meetings, so i didn't feel it too different from what I'd do in the office, such as designs/whiteboarding.

This does require enough capacity from seniors, though.

In short, it can work, though some people may prefer face to face contact, and the organization/availability has to allow for some hand holding.

My first junior job was entirely WFH and it didn't seem that hard. Messages and calls exist. It's not like your only interaction with people is through async PR comments.