There is a bunch of technology for this stuff. You have machine vision inspection systems. There are bed of nails systems that can measure impedance of nodes o

There is a bunch of technology for this stuff. You have machine vision inspection systems. There are bed of nails systems that can measure impedance of nodes on the board and run functional tests. Really high reliably stuff gets x-rayed. If you're doing true mass production you perform inspections/tests at each step of the process.