Parts in the order they constrain each other.
A socket decides the board. The board decides the memory. Work top to bottom and nothing arrives that will not fit.
Pick a processor, see what it locks in
Three parts lists we can ship complete
The numbers that decide fit
The mechanical and electrical interface between processor and board. It cannot be adapted, a mismatch simply will not physically seat.
Different notch position and voltage regulation. The board dictates which one you buy; the processor only dictates which boards exist.
Bandwidth per slot. A Gen4 card in a Gen3 slot works at Gen3 speed, populated M.2 drives can also steal lanes from the GPU slot.
Size the supply on sustained draw plus 30% headroom. Modern high-end cards want a native 12VHPWR cable, not a four-way adapter.
The three measurements that end most builds: GPU length, CPU cooler height and radiator support in the roof or front.
TDP is a cooling design target, not consumption. A 105 W AMD part peaks near 142 W; plan the cooler around the peak, not the label.