Socket and chipset
A processor only fits a board with the same socket, and the chipset has to support that processor generation. A board with an older chipset may need a BIOS update before it will run a newer processor - check which BIOS version it is sold with, or take a newer chipset. One naming trap: the letter E in B650E or X870E means wider PCIe 5.0 support, not an E-ATX board size - the format is always listed separately (ATX, mATX, ITX).
Memory: the board decides DDR4 or DDR5
A board supports one memory generation, not both, and the processor has to support the same one. Also watch the number of slots and the maximum supported capacity: a four-module kit will not go into a board with two slots, and a very large kit can exceed the board limit.
Millimetres: case, card, cooler
The case sets three limits: motherboard format (ATX, mATX, ITX), maximum graphics card length and maximum air cooler height. Liquid cooling additionally needs a mounting point for a radiator of that size. Leave some slack on card length - cables and front fans eat the last few millimetres.
Power supply: watts and connectors
Both matter. The wattage has to cover the calculated load with headroom, and the connectors have to physically be there: EPS for the processor, the right number of PCIe 8-pin, and 12VHPWR (12V-2×6) for newer graphics cards. The unit also has to fit the case - format (ATX, SFX) and length both count.
Drives: not just capacity, but slots too
Before planning several drives, count the M.2 slots and SATA ports on the board and the 2.5 and 3.5 inch bays in the case. A cheaper board often has only one M.2 slot, so there is no room left for a second NVMe drive.
What you no longer have to check
All of these rules run automatically in the configurator: parts that do not fit drop off the lists, and the warnings that remain come with an explanation. That leaves time for what is genuinely your call - price, noise, looks and headroom for future upgrades.