I could view your samples. I really don't see what you are seeing. For FB they look pretty good.
However as the above poster asks, what are the exact settings you are using. You see when you u/l to this site we can see the EXIF data (unless you purposely remove it) . When you post to FB all that gets deleted. FB also renders images to its specifications.
"A lens doesn't cause or contribute to "graininess"." This is why someone suggesting you buy a different lens wasn't a good solution. The Canon EOS Rebel T7 camera has an ISO range of 100-6400, which can be expanded to an equivalent of ISO 12800. As stated it can be expanded to ISO 12800 but using these extreme higher ISO numbers makes you pay a price. That price is increased "grain". It isn't really grain as you would think in a film photograph but the term remains. It is distortion of the electronics inside the T7 because of increased voltage. It is more akin to you turning up the volume knob, maxing it out so to speak, on your stereo and getting distortion from your systems sound quality. As a general rule you probably would want to avoid the top two native ISO values. So 6400>3200 which leaves us at ISO 1600 as the top if 'grain' is your main most concern. Even using 800 is better. On most cameras ISO 200 is the best performer for all around use photos.
A good way to try and test is to go outside on a nice sunny day, set the T7 to P mode and experiment with ISO. Set it to One shot and daylight WB. Tale a dozen or so of random shots changing the ISO manually then go back to PS and do your edits to them. Although I have had several Rebels over the years I never had a T7 but I suspect it is similar to Rebel specs. Your ISO 100 or ISO 200 will probably be your best IQ and the max you can expect from your T7.
EB
EOS 1DX and many lenses.
