Generally kerfing would be used for the face board of the curved arch with the saw cuts facing toward the rough opening framing. There is a technique that can be used where you make a form for the curved piece with a couple of layers of 3/4" plywood and then glue and clamp a 1/8" layer of plywood to the web fingers of the kerf cuts to strengthen the piece a huge amount. This could look like this in process.
The part of the face board not penetrated by the kerf cuts is the portion of the edge of the face board that can be the reveal by the edge of the trim.
The trim itself would not be bent into a curve using kerf cutting. Instead that would be made in a variety of ways. One way is to bend wood with a hot steam approach. Another as mentioned in comments is to glue together many layers of thin wood material. A simpler approach that is often used is to cut pieces of wood in segments and join end to end. These are made up wider than needed and then cut into a curve across the pieces similar to as shown below. It the trim wants to have a non-flat profile that can then be cut after the curve is cut using a shaper or router.
The number of segments that you choose to use will depend upon the look you want to attain with the wood grain. If it is a painted application then you can use longer segments.