It's nuts and bolts and a hose/tubing connection. Or maybe welded. If you can't identify how it is mounted, in person, looking at it, then you're probably not getting it removed under your own power, even with advice from the interwebs.
Rust = Oxidation
Oxidation cannot occur without oxygen. If you submerge bare steel completely in water, it will not rust (theoretically). There is still some oxygen in most water. (And I don't mean the O part of H2O, has to be O2)
Rust "under" the paint means it got oxygen to it. Either from the inside or the outside. From the inside would mean it's already rusted through. From the outside = damage to the coating, or a coating (paint) that was not completely impervious.