They use VPN, Ad blocker and every other apps which people indiscriminately give permissions to, They buyout such apps with significant number of users from the play/App stores and extrapolate the data collected through those apps.

I wouldn't be surprised if publishers directly pay App Annie under the table to boost their stats.

As far as YouTube screen-time is concerned Web, Game Consoles, Media centers & smart TVs would account significant screen-time and it's unlikely App Annie's data hoarding apps are installed in them(Unless say shoddy media center bought from Ali express).

That said, TikTok might well indeed take over YouTube sometime in the future as they are doing what YouTube did in the beginning - Home videos sans shitty cameras, Now YouTube is all about top 1% creators with Michael Bay movie budget for each video.

P.S. Not an endorsement for TikTok, Never used it, Will never use it for their moderation practice against the disabled.