Hi partner,
"can you create a rollup field on Account, of a rollup field which is on opportunity?"
You mean to create a rollup field for the Account entity to rollup all OpportunityProducts of all Opportunities under this Account?
Unfortunately, this cannot be achieved using the rollup field because a rollup over the rollup field is not supported.
If you want to implement it, you have to create a plugin or use a custom workflow.
"Will the rollup of OpportunityProduct on the Opportunity record pick up ALL OpportunityProduct values or just those where OpportunityProduct is related to the Opportunity?"
It only selects OpportunityProducts that are related to the current opportunity, that is, those products that appear in the Opportunity Line sub-grid of the current opportunity form.
In D365, the display name of OpportunityProducts is Opportunity Line.
