There were 3 chips: baseband, RF, and PMIC. The baseband had 2 or 3 CPUs (earlier ones had an ARM 7 for I don't remember what, then they an ARM9 to run the phone and a more powerful ARM11/ARM13 to run BREW, then Android). The RF chip did the radio stuff, and the PMIC did all the power control (Power Management IC). Each baseband chip was optimized for a specific RF and PMIC chip. You could swap them out with what I understood was a lower level of efficiency. As I worked for Qualcomm I was never exposed to non-QC chips.
The display, keypad, battery were generic, use whatever you want. QC didn't make displays, nor keypads, nor batteries.
They also had a single chip line (SC1x/SC2x if memory serves), they took 3 dies (baseband, RF, PMIC) and stacked the dies atop each other in a single package. The idea was to sell them for, I think, $6 each for low cost phones. Add a display, keypad, battery, and case and you've got a cell phone.
I retired when Snapdragon was showing up on my "upcoming stuff" memos.
/ this is greatly simplified at the chip level (e.g. the PMIC let the phone vibrate)
// Wish I'd stayed a couple more years
/// hell, wish I was still there, it was a great place to work. Although former co-workers say that changed 5-6 years ago.
//// retirement isn't what I thought it would be