why old apps not updated since 2015 are trending??
"Trending" change multiple times a day.
Most popular are the ones with the most downloads in the last "x" days" so don't change that often
And yes, there are a whole bunch of apps without a bunch of downloads that are hard to find, but how would they decide what to feature if it weren't for downloads/reviews?
BTW, each timer you update an app in the store, some statistics get reset, such as "downloads in the last X days" as that only uses the current version of the app. So not updating an app as often can help to promote an app.
you can browse all apps ordered by newest first, that's what I do to discover new stuff....
btw trending is not only for good reviews, if you receive 5 one star reviews in a day then you're be the top trending app (currently spotify is one of those that got a few low star reviews today and yesterday and is listed as trending as a result).