What is the primary means by which attackers infect computers with these attacks? How do these attacks commonly occur?

Well ... from personal experience ... that's ransomware ... these attacks usually occur when a HackerMan injects a file that contains the payload ... usually system configuration files ... DLL's, EXE, pretty much any file that contains codes that can modify someone's operating system. This payload then locks your files so that you can only unlock your files using a key that can be purchased online. You enter the key? Your files are unlocked!