There's three approaches I use here.

1. Find some bit that you bonk your head into regularly. Maybe it's query optimization. Maybe encryption or auth. Maybe infra setup. You can usually muddle through it by reading the 5th comment on SE or finding a coworker's working code. But you don't actually understand how or why it works.

2. Think more broadly of where you want your career to be in 5 years, or what you want your next job to be. Map out what you know and what you need to know to be successful in that role. Study the gaps. Sometimes it's not technical skills.

3. Pick a project you find interesting that has nothing to do with your current job responsibility. Frontend? IoT? Systems level stuff? Dig in! You will find that unrelated computer stuff is in fact all related in some way.

The least adventurous approach is to work at the edges of what you're already doing. Are your apis usually consumed by react components but you've never written one? Try writing a react app that consumes one of your services and see where the pain happens. Even if you never use react again you'll have learned something about your work from a new perspective.

Don't spend too much time on anything that isn't fun. Chase joy and fail fast.