Hi In your first example it says 'lean on' not 'learn on'. To lean on someone is a phrasal verb =depend on/gain support from someone. In this example it means use their experience to help you. Your second example 'to learn on the job' is different. It's not a phrasal verb. It is the verb 'learn' and a separate preposition. As far as I know 'to learn on' doesn't exist as a phrasal verb

Learn on isn't a phrasal verb, but "on the job" is a phrase. For example, I'm going to have on the job training, starting on Monday.

To learn on the job means you learn by doing when you are employed, rather than beforehand. This is different to learning about the job, which uses the preposition about. The preposition here changes the meaning of the phrase.