This is completely incorrect and this mistake is what scam projects have used to scam newbies. STO tokens are not securities. The SEC does not recognize and cryptocurrency as a security and therefore the term is a wrong application. I suggest you to edit this part of the OP to make sure no newbie falls prey of STOs anymore.

Although they no longer exist since SEC made its stance clear, they quickly moved to IEO and IDO.

Another big source is private funding which is unknown to most people who enter this sector but a very important factor which makes a lot of impact. It is the reason why the projects dump and never come back up again. Advisors/VCs pay to get a locked in amount of tokens that are sold at a specific time period unknown to public funding. Hence any project that you try to get into, it will end up as a failure if this period it not tide over. In most cases they cant and they go bankrupt once advisors pull out.