Custom solution Vs default solution

Dear all

I don’t really understand the relation between custom and default solutions, in my case, I have created a custom solution and I have added/created all the new/existing entities I need, I also have created some processes, what I don’t understand why these processes are also available in the default solution?

I also need to know what happens if I delete these processes from default solution, will this make them deleted from the custom solution as well?

Second point, I started using CRM configurationMigration tool, I created a schema and I added all related entities (all), but when importing the data on the new destination instance, I received hundreds of errors, mainly related to dependencies.

Any idea or resource to help me use it professionally? Also is it the right tool for migrating booth DATA and CONFIGURATIONS?

Thanks.