merge albums in Photos App

Quick outline of one way to merge album A into album B:

Make sure the left sidebar is visible. Scroll it so that album A is visible.

Click on album A to display its contents.

Select all photos in album A.

Scroll the left sidebar so that album B is visible, but do not click on it.

Drag the selected photos into album B. (They will also remain in album A.)

If you no longer need album A, scroll back and remove it.