1) Here's a 3minute video showing an analytic function being used to remove duplicates

https://www.youtube.com/watch?v=6MgU6Cnqy-0

2) Yes you can, it can be in any plsql block of code

3) For views, it depends if we can sensibly map the update to the objects that make up the view. This is concept is called whether the view is "key preserved".

Some info on that here

https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:548422757486

For materialised views, do you mean whether it can be fast refreshed ? or actually updated ? (which is a different topic)