The "Start an approval" action needs a "Delegate Users" field (or named something similar), which allows users/groups nominated in this field to:
View the outstanding approvals/approvals history that have been assigned to other users, regardless of who has been assigned
Approve/reject an approval on behalf of one of the nominated user(s)
Something similar to this:
The users nominated in the "Delegate Users" field do not receive an email when an approval is assigned to the user(s) in the "Assigned to" field - the only way they would see them is through the "Approvals" page in Flow. This means the Approvals page would need a "My Requests" and "Other User's Requests"/"Delegated Requests" (or something similar) so that the logged on user can see which approvals have been assigned to them, and which approvals they are a delegate for.
This is similar to the need for administrative control over approvals (see my other post here), however provides for more granular control.