Their website mentions that “both 3-pin and 5-pin MX mechanical switches” are supported. 3-pin switches are also known as “plate mount” switches and 5-pin are also known as “PCB mount”. I’d recommend getting a switch tester / sampler off Amazon to get an idea for what switches you want (linear vs tactile vs clicky). There’s a lot of switches out there and most of them will fit the keyboard because it supports 3 and 5-pin. I personally like Boba U4 silent tactile switches.
I’m not sure about the keycaps. Keychron’s website also mentioned that their RGB works with OEM or Cherry profile keycaps, but I’m not sure if they have a specific layout on the bottom row that you need to take into consideration.