Writing 'testable' methods in Angular

For test preparation, the coding guidelines state that methods must be written in the developing language so that they can be tested by unit tests.

After a lot of research, I understood the topic for Angular as if the functions in the .ts files do not actually have to be adapted for the .spec.ts files.

This video explained it most clearly to me: https://www.youtube.com/watch?v=BumgayeUC08 but I also understood this from various searches.

Have I really understood this correctly? If not, how can I ensure that a method can then be tested with unit tests?