Learning SQL early on in uni. I attended a course where you passed the course if you completed a certain percentage of the home exercises on a web platform. They were just tons and tons of different kinds of SQL pickles against a fake corporation Postgres database, probably more complex than you'd ever need in an average CRUD app project. This was very useful in my first job were I had to make sense of a weird legacy data platform built on top of MSSQL but also later on in basically any project ever involving a relational DB.

Git somes to mind too, where it changed the way I think about how to do the day-to-day writing of my code and that the knowledge is very transferable across all kinds of projects.