Mid sized companies do 1. Retargeting 2. Demographic based ads(especially location based)
Big companies do branding.
Retargeting: If a user visits your website, you could ask google to add a tracking cookie to that visitor. You add additional information about that user like which stage of the funnel the user was in(eg., visited a product detail page, visited pricing page, went into checkout and dropped out, etc.). Based on this you could target a specific version of ad to that user, wherever he goes.
Most ads I see on Youtube are from companies that are in growth phase doing retargeting. Eg., Legalzoom, Zappos(and a few other e-com companies)
Demographic targeted ads: I saw that the set of ads I'm being shown have been changing drastically based on my location. And the ads I see also are different for me vs my friends who are slightly older. They mostly get insurance ads while I get beer ads.
To re-iterate, all this is empirical observation, and I don't have a first hand experience about this. But hope this helped