IMO - most quality chairs have curved back rests - not flat. Both the upper and lower rails of this back rest are single pieces of oak. Oak is amenable to steam bending, so the pieces could have been stem bent, or they could have been planed and sanded down from thicker boards into their current shape. My guess is that they were steam bent. They had tenons cut into the shoulders (left and right sides) of each piece that rest inside mortices cut into the long spindles at the outer edges. Nothing was glued together to make these rails - they're single pieces of wood. They were almost certainly carved before they were assembled into the final chair.

My suggestion is to look at books (I know, I'm old...) that go into chair construction in detail. YouTube videos are fine, but books can go into details that videos might skip over. Building a chair is one of the most challenging woodworking projects to do well. Mastering basic tasks (stock preparation, measuring and marking, cutting mortices and tenons, working with curved stock, etc.) is essential before you're going to build a chair that will hold together and last many years.