Customer managed ones are optional, but many tutorials include them as that's standard security for lots of companies. I have made the mistake of this and cost Databricks $1200 🤡
If you start deleting your network pieces, it'll delete things associated with it
Recommendation: take a look at the docs linked. If it looks like you set up a managed one, you'll have to recreate everything with the default in a new Databricks workspace (sorry)
As for the AWS charges, I couldn't guarantee what's part of the free tier and what isn't. I do know their support team is quite good so if you drop them an email they should be able to give you guidance.
