I feel sorry for whomever will have to clean that disaster (without even having their pizza). Better replacing the oven entirely than trying to remove burnt cheese from the bottom.
It’s not that hard just put a baking tray with soapy water on the top rack and blast the oven as high as it will go. There’s still a lot of scraping and scrubbing but it makes it easier.
Exactly this happened to my mum when she tried making homemade frozen pizza without prebaking it. Either this is a store bought frozen pizza that thawed in a serious manner or something similar happened.
Source: I sustain myself on a healthy 33% Pizza diet.