If you can get to a bed shop (Ikea, next home, dreams, bensons, dunelm, to name a few, but google your local shops) that will mean you can browse in person what you like and don’t like. This will also help if you’re thinking about downsizing to a double as you’ll need a new mattress and in store you can lie on the mattress and find one that’s comfortable for you. If you can’t get to a store, all of them have websites to browse and get a feel for what you like. Other websites include Dusk, Get Laid.
Frame beds will be metal or wood with slats, and will come in a variety of styles that you can store things under, either because the bed is high enough off the ground to fit things under or it has a storage element like an ottoman bed. These kind of beds would be delivered flat packed.
Divan beds (the ones with a flat base with drawers under) usually come as two parts, like half a bed each, that fit together with clips or bolts then the mattress goes on top.
With both kinds of bed the headboard is usually bought separately unless it’s a wooden frame which has the bit at the top and bottom of the bed like this 🛌.
Separate headboards are usually attached onto to the back of the bed, with the bed designed with a place for it to fit and be screwed on to save you having it on a wall.
No matter which type of bed or headboard you get, you’ll likely need to pay get it delivered to where it needs to be which means it’s on the company to get it up your stairs. Some companies also offer an assembly service, or you can buy that service somewhere else (like task rabbit).
Hope that helps a bit with the overwhelm - even a simple task can feel a lot of you’ve never had to do it before!