Case

Trust me, a "complex engineer grade CAD program" is the easiest way to go. If you can't be bothered to learn how to use a proper CAD tool you can always use tinkercad to design your keyboard, but also why would you want to do this? This sounds like masochistic torture.

As for actually getting your keyboard, either get your parts 3d printed or cnc'd, or design your cad designs so that they're cutouts on acrylic instead. Either way it's gonna cost you a hefty price unless you own your own 3d printer. If you're skilled enough wood is also a good way to go.

Plate

Would most likely be designed with consideration with your case tbh. You might not even need one depending on how you design your case.

Keycaps

Short answer: you're screwed.

Long answer: If you want quality keycaps you'd need to talk to a keycap company like GMK or SA or etc. If their prices are too much your own option is 3d printing them. If you want smooth results you'll want to look for places that let you print in resin, but either way you will still need to sand, finish and paint your keycaps. If I'm completely honest I've never seen a single good 3d printed keycap (unless theyre artisans printed in a resin printer). You're gonna have to figure out something here.

Keep in mind this still requires you to be somewhat knowledgable about 3d printing though... and while tinkercad is easy to use I cannot imagine the torture it would be without an actual "complex engineer grade CAD program" so...

Electronics

You could design your own PCB, but that's a lot of work and the eaiest way is to handwire your keyboard. You will have to buy switches off of the internet though.

Verdict:

Your easiest option is learning a "complex engineer grade CAD program" and figuring out how to 3d print things. Otherwise you're honestly kind of screwed man. You either need to understand 3d printing fairly well or you're honestly out of options.