Excuse me, why are the photos taken by SLR in the studio very dark?

To put it simply, you can use manual M-stop, the aperture is around F8, the sensitivity is 200-400, and then take a Shoot. If it is too dark, it will increase the shutter time. If it is too bright, just try one gear at a time until you are satisfied. Do not use automatic stop. The metering mode of the camera itself is not accurate. Better have a tripod. You can highlight the theme with the largest aperture.