I've really liked hitbox controllers for Street Fighter 6. I've been really happy with how they've turned out and they're a joy to use. If there's interest I can post the build process/instructions/guide.
They are arcade sticks that, instead of using a joystick with 4-directional switches, use 4 buttons for the directions. They are often called "Hitbox" because that's the brand that popularized them. Some people also call them "all-button controller" or "stickless controller". The advantages are ergonomics (many people have developed carpal tunnel syndrome from traditional joysticks) and economy of motion (it's easier to press 4 buttons with 4 fingers than move the entire wrist/forearm to move a joystick).
It's pretty much like using a keyboard for fighting games, but you only have the buttons that you actually use, the keys are large buttons, and there are no issues with pressing many simultaneous buttons (many keyboards have problems with that). People also often use PCBs that are compatible with PS5 or other consoles, and also SOCD cleaners (which is a piece that helps configure which inputs should be sent if you press "Simultaneous Opposite Cardinal Directions").
They've been gaining more and more popularity over the last few years. If you are good at typing, you may be more comfortable playing fighting games with this than with a traditional joystick or even a gamepad.
I'm not sure about why that's the standard, but it may have to do with the fact that in fighting games you don't want to be jumping all the time, it's something that you have to do carefully, while crouching is something you do constantly, and most motion inputs involve the "down" direction (like quarter circles or the shoryuken input), and having left-down-right lined up makes it pretty easy to do those motions. Think about pressing your Q-W-E keys in quick succesion. You can probably do that without much practice. That's a half-circle-forward motion. If you have "down" in your thumb, a half-circle would be like pressing "Q-Spacebar-E" in quick succesion, which is more difficult.
Maybe it's also because people often use the space bar in keyboards to jump in many games, and that's what they were used to.
In any case, once you get used to this layout, it's very comfortable.
The down button is used constantly and used with the directions to input quarter circle motions etc for special moves. It would be very awkward to have down under the thumb. Up should be used much more cautiously and purposefully.
Other layouts are possible and some people find the WASD layout more comfortable. Some even end up using keyboard buttons for the left hand in WASD layout with arcade buttons on the right (a.k.a. mixbox).
The person commenting above you seems surprised by this "all button" design, but this is becoming very popular for its precision compared to stick or controller. And it's no different using WASD keys to walk around in an FPS game rather than using an analog stick on a controller. If you play fighting games I'd highly recommend trying a leverless controller like this.
I was curious too so I looked it up. Apparently, the 3 buttons on the left are I suppose left, down and right, and the others are for attacks. It's a specialized keyboard for fighting games.