I recently bought an a6400 and sigma 18-50mm f2.4 as my first camera. I was looking for a somewhat compact camera that would give me better photos than my smartphone (2021 flagship phone).
I really like it because I am able to throw it in my regular day backpack and the photos are much higher quality (image quality wise) than my phone.
I knew that APS-C would perform worse in low light but I didn't think I needed to worry about that because I assumed low light meant nighttime... I didn't realize low light meant indoors...
I took some indoor photos today and a ton of my photos came out grainy or blurry. I shot in Aperture priority mode at f2.8 and limited ISO to 2000. The camera would usually max out ISO at 2000 and select a shutter speed of something like 1/80. Shutter speed was sometimes too slow for a sharp image so I went into manual to lower it and then the images came out too dark.
Is this a APS-C "limitation" or am I doing something wrong?

tl;dr - Bought APS-C camera, did not realize indoors was considered "low light", indoor photos come out blurry OR grainy. Am I a retard using the camera incorrectly or is this a APS-C limitation?