CPU socket and chipset
CPU and motherboard must share a socket, and the chipset must also support that CPU generation. Older-chipset boards may need a BIOS update first, so find out which version the board ships with or go with a newer chipset. Watch for one naming trap: the E in B650E or X870E signals wider PCIe 5.0 support, not E-ATX sizing. Form factor is listed separately as ATX, mATX or ITX.
DDR4 or DDR5: set by the board
Boards take a single memory generation, never both, and the CPU has to match it. Count slots and check the capacity limit as well, since a four-stick kit can't go in a two-slot board, and a very large kit can go past what the board supports.
Clearances: case, GPU and cooler
Your case imposes three limits: board form factor (ATX, mATX, ITX), maximum GPU length and maximum air cooler height. An AIO also needs a mount for its radiator size. Don't cut GPU length close, as cables and front fans use up those last few millimeters.
Power: enough watts, the right plugs
You need both. Wattage must cover the calculated load plus some headroom, and the cables have to exist: EPS for the CPU, enough 8-pin PCIe, and 12VHPWR (12V-2×6) on newer GPUs. The unit itself must fit in the case too, by format (ATX, SFX) and by length.
Storage: count slots and bays
Planning on multiple drives? Check how many M.2 slots and SATA ports your board has, plus the 2.5 and 3.5 inch bays in your case. Budget boards often stop at one M.2 slot, leaving nowhere for a second NVMe.
Let the configurator handle the rest
It applies each of these rules for you: mismatched parts vanish from the lists, and any remaining warnings explain themselves. You get to spend your time on the real decisions, like price, noise, looks and room to upgrade.