My first job inside a real engineering team, before GTM and before Clay, back when shipping meant someone else had already written the tests.
React components built against an existing unit-test suite, plus a long run of bug backlog cleanup across the product.
How to work inside a team, mostly, and inside a large codebase I had not written a line of. Learning other people's patterns, matching the design system, changing code without breaking what surrounds it: that is a skill you only build in a codebase bigger than you are.
The other half was collaboration. Picking up tickets, asking questions in the right place, taking feedback on a diff without taking it personally: that part carried into every role since.