This is what every pan in 90% of commercial kitchens look like, because aint nobody paying someone to sit thete and scrub them. They get run through the sanitizing dishwasher and then they're 'clean enough' and any other bits get burnt off/into the pan surface. So people saying this pan is clean arent really wrong, especially since the carbonized bits are almost definitely inert and non bacterial.

Just put parchment paper on it

I have heard of seasoning a cast iron skillet but never a baking sheet.

Be an artisan and clean it well. Your time is only wasted if you dance the capitalist watusi, where somehow, time is money. If you don't want to clean it, that's acceptable as well.

The idea is to build up enough layers that you can put a glaze like cast iron on it.

Parchment paper is cheaper and far easier to work with.

Foil is for messy things that would leak under the paper.

Also you can put it in your oven and put it into self-clean mode.

Also basic powder dish washing detergent with a little water and time.

Alternating those with a rinse between is fine. Using them together just makes salt water.