Salesforce 1gp vs 2gp

For 1gp we need a packaging org.

For 2gp we do not need a packaging org. (need to use Developer Hub - but that does not support Security Review - must use a developer edition org).

Salesforce 1gp cannot extend 2gp. So, as soon as you define a namespace in an org, you cannot install 2gp package in there anymore.