A lot of blue screens were caused by unstable graphics card drivers. Thank Microsoft for moving a lot of the graphics stack to userland in, I think, Vista -since then, GPU driver crashes don't take the entire system out as well.
A lot of BSOD were also caused by generic and low quality DRAM sticks, unstable individually (failed addresses) or becoming unstable when paired in dual channel setup, especially with 2 pairs of dual rank, dual sided sticks. Had very often this issue with Corsair Value sticks, or with a XMP (Intel spec) RAM kit used with 1/2/3th gen Ryzen, manually setup to match the 1.35V XMP profile (for DDR4). EXPO does this much better now.
Diag tools like Quicktech Pro then Memtest86+ from Passmark Software and the free one were born on this 'lost era'.