User ID questions

I am new to auth0 and have read a lot of docs but still have some confusions: Is there a good explanation regarding the User Id and how it is returned in the login process? It is unclear to me when a ‘user_id’ or ‘sub’ is returned and also can you explain when a namespace is used and the pro / con of the namespace - is the namespace a requirement, or best practice? Also, when the namespace is in play, what is the best way to parse / access the user Id value? It seems to come back in the form: namespace URL Auth0| which in my case is: Auth0|33434343443, so is everything to the left of pipe symbol ‘|’ the IDP and everything to the right is the user’s unique Id? And lastly, is this value encrypted by default and if not how to turn on encryption?