Forgot to add - many larger companies offer a junior pm program to help people transition into a PM role. I work at oculus and we participate in Facebook's 'rotational pm (rpm)' program.

Applicants have little to no product experience and are sometimes right out of college or other times come from different fields or backgrounds. Once hired, rpm's rotate between a few teams and are assigned a full time PM mentor who works alongside their manager to set them up for success.

I believe Google has something similar as well.