I learned a ton from contributing to large open source projects.
Find some software that helps your company, and don’t be afraid to dive into the codebase. There may be times you can even contribute as part of your job.
It is unfortunately not efficient compared to what you get when working with actual colleagues within a company.
Open source projects maintainers very likely have their own day jobs that are unrelated or only marginally related to the projects, have different priorities or just don't care enough. A previously important feature that has seen lots of activity may be on the shelf, but that's only known among maintainers with no public notes. You don't get to go into meetings or just send a Teams/slack message to get a quick response, let alone a casual chat at the coffee machine.
Unless you are working on prioritized features on a project like linux, VSCode or Chromium, chances are that your issues or pull requests go in the log until someone works on it months or even years later.
Find some software that helps your company, and don’t be afraid to dive into the codebase. There may be times you can even contribute as part of your job.