File to media migration: use node title for media title
Without knowing your content model, you can't answer this question directly, but what you'll need to do is query your node title for every fid entry for your source file entities table. You can do that in many places (e.g. source/process plugin) with your existing source plugin being a good place.

Using your custom file_entity source plugin as an example, you'll need to update the query() method so  its DB query joins the node table based on how you're content is modeled (e.g. entity/paragraph references) then selects the title column so it's included in the row data. You can follow that up with updating the fields() method so your source plugin will provide the title field you need.