> Why is os and phone model so tightly integrated and locked together, when in the pc world you can generally install any os on any computer?

Qualcomm is the reason. Qualcomm wants OEMs to buy new chips every year, and for that to happen, consumers have to buy new phones every year. To upgrade Android (across kernel versions), OEMs need Qualcomm to provide updated drivers, which Qualcomm has been reluctant to do, because their sales will be undercut by chips they sold years ago. Android phones are closer to Mac than PCs, as there's one hardware-maker who determines which models become obsolete, and when, based on the software they choose to update (or not).