My toilet seat is loose

There should be a button to press which will release the seat from its hinge post on either side. If it's not the large silver knob then it may be hidden:

The pic below is from a toilet similar to yours, and may help with figuring it out. It shows how the seat hinges slide and clip onto the hinge posts mounted on the toilet:

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Here's your problem right here.

There'll be a vertical screw going straight down through the porcelain toilet bowl. If you follow that imaginary vertical line, you'll find some sort of recess or pocket providing access to the "nut" on the bottom.

Expect a plastic cartoon version of a nut that is designed to be tightened by hand; it's normal for those pockets to be confined enough to make it hard to get tools in there. Or if not the pocket, the working space lol. Sometimes the nuts get sticky and need some persuading, but improvise what you need - I've used a huge 1-1/8" hex socket that fit over the plastic nut, I've used pliers, etc.

DO NOT go down to the hardware store and get actual metal nuts of that same thread. The cartoon nut is plastic for a reason; using metal hardware will easily crack the porcelain and then you need a new toilet. Been there, done that (the guy before me had selected metal hardware; I just tightened it without thinking).

Now you're saying "I know about that loose nut; I choose not to fix it and want to fix the problem I stated" -- but they're related. One is causing the other.