Here is the best link I have found that answers several questions, if you read through there are links to also click in the guy who answers the questions in his signature:
https://forums.iboats.com/forum/boat-repair-and-restoration/boat-restoration-building-and-hull-repair/503104-replacing-a-transom-on-an-aluminum-boat
I'm sure many of the old boats used a solid piece of wood. I do know that one type of oak is better than the other, but can not remember if it's white or red that is better to use. Plywood has advantages in that the multiple layers make it much stronger, also from my reading.
Here's a video I watched before I cut out and glued my Transom Boards together:
https://www.youtube.com/watch?v=P3bFc2UjNao
You do not need to fiberglass your transom as in the video, but at least this gives you an idea how to pattern, cut, and glue your boards together if you go the plywood route. I would search on youtube and watch as many videos as you can to see how many ways people glue and clamp the boards together. You can also search Google images and see many different transom projects, with boards held together with clamps, screws, bolts, etc.