In addition to incognito, also Chromecast. A Chromecast will insert ads before videos added to the queue by non-premium accounts. As a result I got a family account so I wouldn't have to watch ads before videos my wife enqueued.
I should note that I've had Youtube premium on all the accounts in my household for a few years now, so it's possible things have changed since. My guess is that it would have only changed in the direction of "more ads" though, not less.