Introduction: How to get into keyboard building?
Keyboard building is a very broad topic. Some people change keycaps and call the result a custom keyboard. Assembling a board from a case, stock PCB, putting in switches and caps is another common way of customization.
However, a real custom keyboard IMO is tailored to your fingers and your typing habits. Doing the research, designing and manufacturing your own keyboard is really rewarding.
Anyway. Whether you are interested in changing caps and switches or designing your endgoal from scratch, this hobby offers you a lot of opportunities.
Best way to get into the hobby is probably lurking on r/mk, r/ergomechkeyboards, geekhack etc. for a while. You have to get familiar with the basic terminology and your options before ordering anything.
At the beginning try to ignore overhyped stuff (most group buys) and focus on what really matters (basic switch types, keycap profiles, form factors and physical layouts).
Once you know what you want, you are ready to build.
But your hardware in only one half of the equation: logical layouts are just as important. Alternative layouts like Dvorak, Colemak etc. and tools to customize your keymap can make your life a lot easier.
To start building look for build logs as a reference. They helped me a lot and I've collected and written some.
Building a truly custom keyboard in 2-3 months is realistic even with zero knowledge at the start (research, waiting for orders to arrive etc. included).
I've done it during an autumn.
If you are still with me, here is a detailed learning plan for keyboard builders.