I consider media outlets mainstream when they are all owned and controlled by one of the big groups like Sinclair. E.g., if they've ever made it into one of those montages where all the anchors from all over the country say the same catch phrases. I think they're different, because they anchors cannot say what they know to be true, but must stick to the script even when they know it's wrong, or they get fired. The smaller independent outlets can push propaganda as well, but they're not forced to, and their outreach is a small fraction of the conglomerates.
> I called out mainstream media, as people often assume it is more reliable than the non-mainstream media.
It's pretty ambiguous: Where do you draw the line between them, and what evidence do you have about their relative reliability. The professional journalism I see, e.g. news sections in established newspapers, is far more accurate and honest than the non-mainstream stuff I see.
I believe non mainstream media is easier to recognize as propaganda or at least heavily biased, while mainstream media is oftentimes considered "unbiased" and "objective".
It's quantitatively different. Propaganda is most effective when its reach is maximized because having it appear in all mediums gives it the appearance of legitimacy.