Let me help
2 approaches to using a typical camera as a video source
1. HDMI output
2. USB
there are other options but rarely for consumer grade equipment, so I'll stick to those 2 for now
No, you do NOT have to use USB for connecting (DSLR, in this case) camera to PC. Like so many things technical, the answers is ... it depends
For HDMI out, the issue is something referred to Clean HDMI out, ie without camera status/settings info
- many older and/or lower-end cameras, are not designed for Clean HDMI out. This would be a camera setting, and model/firmware specific issue. Many newer cameras, taking using camera as webcam/video device into account, now have a Clean HDMI out mode
This is controlled by the Camera, not OBS. All you could do in OBS is crop out the outer edge to eliminate the camera settings info if the camera doesn't support doing so.
For the Canon, there are numerous camera forums with discussions on best options (per model camera) on using inter-changeable lens camera as a webcam, if practical (and isn't always)
For USB, the issue to be aware of is bandwidth. USB2 can't handle uncompressed 720p traffic, so when using recently released USB Webcam software (from Caon, Nikon, etc), on older USB2 interface cameras, you will get a low-resolution image only over USB. Why does a typical USB2 webcam not have this issue? it compresses video on camera device (webcam) and the driver uncompresses signal on the PC.
So, Check out the Canon EOS Webcam utility, but don't except 1080p, or even 720p resolution. Canon's software documentation and FAQ goes into more detail.
Now, your best video quality would come from recording on camera, then loading that video file onto OBS PC and setup as OBS Media Source , then using that pre-recorded video and edit/overlay, etc in OBS. but that isn't what you want, and takes more time.
So do a (google) Internet search on "Canon 250D clean HDMI out" to get best resolution live video feed from that camera. Be aware you'll need to purchase a Mini-HDMI out to HDMI cable, and an appropriate HDMI capture card (based on resolution 1080p or 4K, typically) and PC interface (hopefully your PC has at least a USB 3 interface).