Xbox Series X uses a dynamic resolution with the highest resolution found being 3840x2160 and the lowest resolution found being 1920x1080. Pixel counts at 1920x1080 seem to be rare and pixel counts at 3840x2160 seem to be very rare. Xbox Series X appears to be using FSR 1 to upscale to 3840x2160.
It seems that Xbox Series X tends to render at a higher resolution than PS5. As an example, during one scene PS5 rendered at approximately 2418x1360 and Xbox Series X rendered at approximately 2581x1452. However, there were also a few scenes where they were rendering at approximately the same resolution.
Xbox Series S uses a dynamic resolution with the highest resolution found being 1920x1080 and the lowest resolution found being approximately 1152x648. Pixel counts at 1152x648 seem to be rare. Xbox Series S appears to be using FSR 1 to upscale to 1920x1080.