My toilet has been caking up with filth which won’t go away with a brush. Any tips on how to get it white again?

This toilet has been a source of embarrassment for months now, I’ve tried scrubbing with multiple of the toilet cleaning brush pads which only get some of the discoloration off. My building is very old and sometimes it seems like waste water comes up to my apartment and urine will have been sitting in the bowl over night even if I haven’t used the toilet/flushed before bed which presumably makes the staining worse.

It’s just such a bummer to look at and I’d love for it to get to a nice clean white, but it’s almost like it’s cemented on the porcelain. This is my first time living with hard water, not a fan at all