Q1: Databricks repos uses Git (or Azure devops or Gitlab etc) so Databricks repos is just an umbrella term.

If it will work with streaming? Yes. The functionalities of Repos is limited (read: code versioning using branches). If you need prehooks f.e., that is not possible.

You do not have to use Repos, you can work without any git provider as Databricks keeps code (notebook) history itself, although I highly recommend using Repos.

Q2: you can use Azure Devops or Jenkins but it is not a requirement. If you have a small/simple environment, CI can be overkill. But if you need to use automated pipelines, yes that is possible.