Mechanical Keyboards
Believe it or not, this is actually a hobby! Mechanical keyboards are keyboards that have a mechanical switch for each key rather than a rubber mat with bubbles on it. You can customize pretty much every aspect of these keyboards if you get the right one, with some people even making their own from scratch!
The Board
First, you need a circuit board. I'm including the case, switch plate, and mounting system in this section as well because I've never customized those myself and most entry-level options will have all 3 parts. If you don't know how to solder, you want something called a hotswap keyboard. Look for one with 5 holes for each switch. The board that I have are the RK ROYAL KLUDGE R75, the Epomaker TH80 (I think, this is the closest I could find), and the original GMMK (the latest version is linked). This is usually where the bulk of the cost lies: the latest GMMK costs $270 without any keycaps or switches. However, the entire R75 cost $60, so you can definitely start with a cheaper option.
Direction
First, you need to consider whether you want your switches north-facing or south-facing. For most people, it doesn't actually matter. However if you want RGB to shine through the legends (letters) of your keycaps, you'll want north-facing, because the LEDs will be at the top of the key. If you want to use keycaps labeled "cherry" profile, you'll want south-facing, because the keycap will hit the switch before it's pressed all the way sometimes if it's north-facing. For me, it's a bit of a non-issue as I don't care about either of these. The TH80 is south-facing and the others are north-facing
Case Material
Case material choice is all about sound. The keyboard will usually sound deeper and more resonant if the case is metal, especially the bottom. Metal also makes it look fancier.
Mounting
I don't know as much about mounting systems, since I can't customize them on any of the boards I have. The latest GMMK has gasket-mounting and offers two options for different amounts of flexiblity. If you pick a more flexible mounting system, the keyboard flexes when you type, which is apparently good if you type for long periods of time. I use my keyboard mostly for gaming though, so I prefer a stiffer keyboard.
Switch plate
The switch plate is the layer that sits between the edges of the switches and the circuit board. Different material provide different sound profiles and flexibilities, with metal being better for sound and plastic being more flexible. Some switch plates will even have slits cut into them to make them more flexible, but those are usually only seen on high-end hobbyist boards.
The Switches
The switches are my favorite part of the keyboard to customize. Different switches have different sounds and feel different under your fingers. There are many kinds of switches available for purchase, and they can be further customized by lubing or "filming" them (more on that later). There are three categories of switch: linear, tactile, and clicky. Linear switches feel smooth to press, while tactile and clicky switches have a bump on the stem that you can feel. Clicky switches also have a plate that makes a noise when the key is pressed. I recommend getting a switch tester with many kinds of switches so you can try them out and see what you like. Hippokeys offers some pre-selected options, as well as allowing you to buy single switches of specific types. Note: when outfitting a keyboard, you should buy in packs, not singles, since it's cheaper.
Travel distance
One main consideration for your switches is how far they need to be pressed before the letter appears. Shorter distances allow for faster typing and gaming keypresses.
Springs
The weight of a spring represents how much force you need to press the key. For speed, you want something light. However, if you like to rest your fingers on the keys, it can't be too light. Especially heavy springs can be useful for people with tremors so keys don't get pressed accidentally. Some springs are also graduated, so you apply less force at the top of a keystroke and more force towards the bottom, and some springs are 2 or 3 stages which also changes the feel.
Tactile bump
A tactile bump is a literal bump on the switch. Usually, but not always, the bump is at the actuation point; aka when the letter appears. Sensitive typers can feel this and only press until this point instead of "bottoming out" or pushing the key all the way down. The bump can be large or small and sharp or rounded.
Material
I don't know a lot about this, but supposedly different materials for different parts of the switch will produce different sounds.
Customization
There are two main things you can do to a switch: lube it and film it. To do either of these things you need to take the switch apart. You use one of these or just shove a thin tool under the little latches on the side. If your keyboard already came with switches, you'll need a keycap and switch puller like this one to get them out of the keyboard. Once taken apart, you can use a tiny paintbrush and some switch lube (different kinds for linear vs tactile switches are available on hippokeys). You can find guides on YouTube that show you where to paint. To "film" a switch means to put a little sticker in between the top and bottom casing to achieve a tighter fit. You use these guys right here. You'll probably need a tweezers to apply them. Lubing helps the feel and sound of switches, where filming is mostly about sound. Other customizations include swapping out different parts to make "frankenswitches," the easiest part to swap is the spring.
Issues with switches
There are sometimes issues with switches. Sometimes it's an individual switch, sometimes it's that model of switch. Two common issues are stem wobble and scratchiness. Stem wobble means the keycap will move around a tiny bit like a joystick, and no, that's not fun. Scratchiness is just a not smooth-feeling keypress, and can usually be alleviated by lubing.