Micro balls /bumps/pillars, not solder nowadays for the most part but making the interface to the substrate and bonded in epoxy or other exotic bonding techniques
Everything is balls/pillars for interfacing from die to substrate to PCB
As for creating those bumps on silicon
Did that on one hand on cellphone while baby is sleeping on me, sorry if it’s barebones hehe, but from that you can search the subjects deeper.
Edit, one of my favourite channel on the die technology, brief history of packaging