1. Filling in the lookup field for the first time, before or after creating the record is changing the parent from "nothing" to "something", so this is a reparent action, exactly the same as (2)
2. When the parent of the Opp is change by filling in, or changing the lookup to the parent Account, then the cascading behaviour for "Reparent" determines whether or not anything happens. "Cascade All" or "Cascade Active" will add an implicit share to the Opp granting access for the owner of the Account, and for the Access Team of the Account (if there is one already), and for anyone that has an explicit or implicit share on the Account.
Owner gets full rights (but their privileges might trump this - eg if Alice (the owner of the Account) has Delete:None for Opps, then they can't delete this Opp any more than they can delete their own. Basically they can do anything to this Opp that they could do to it if they owned it, even though they do not.
Shares from the Account (whether user, team, or Access Team) are copied with the same shared rights down to Opp. Again, privileges might overrule these - if Bob has Delete:User on Account, and this Account record is shared with Bob with Delete rights, this will cascade down to the Opp. But if Bob has delete:none on Opps, he still can't delete it.
All of this is done by reparent: the only thing that changed is the lookup, not ownership or sharing or anything else.
3. Access Teams use sharing. The system creates a Team, adds a user to it and shares the record with the Team. Sharing cascade rules determine if this is cascaded down the Opps or not.
If Cascade:All, the Access team will have shared rights to the Opp the same as to the Account, which means that users in that Team have those rights too. Cascade:Active will only affect Open Opps. Cascade:User Owned would only share Alice's Opps, not Charlie's (under the same Account).
4. If you have Assign cascading, then the Opps will also get assigned. If you have reparent Cascading, there is a cleanup here - Alice will no longer have rights to the Opps she does not own. The new owner does not gain any rights.
5 Account>Account changes, yes this is a reparent, but a reparent of the Account in the Account: Parent Company / Account relationship. So this would give rights to the owner or sharers of the parent account over the child account. This might then cascade down to the Opportunities, depending on the Account:Opp relationship cascading behaviours.
Phew!
