What is the procedure for replacing a new-build vinyl window with drywall returns?
Its installed by a nailing flange on the outside, it is covered by the window trim. Remove that and you will see the nails holding it in. The window jamb dimension is the outer diameter of the window where it meets the exterior trim, the RO is a 1/2" to 1" bigger than that.

There is a small chance it is held in by nailing tabs, attached to the round jamb and extend into the interior side of the RO. Set with nails in the RO to hold it in place, I seriously doubt this method is used.