There is a built in Download Manager in Android which was used by many system components and apps. For example, Google play Store uses it to download and update apps, and Google play services uses it to download system update, and Google Chrome uses it to download pwa menifest(I assume). However it can only proceed one task at a time. So when one task is in downloading status, all others will be in pending status.
I am using a Pixel 2 XL.
Starting from Android 8, I often see a strange situation that all update tasks in Google play store are stuck in pending mode for days, and I can not download any other apps nor system update at all, I can’t even add a PWA website to my desktop. (At Google DevFest 2018 SHANGHAI, I submitted the PWA issue in Q&A pannel, and the Googler who’s giving speech about PWA don’t even know why.)
Until one day when I flipping through my installed app list in play store, I found one app is stuck in configuration status, and there is no way I can easily find it on top of any app list, then I figured out why my download queue is always stuck.
So to solve this download pending problem, all I can do is to find out what is stuck in download queue, and cancel them manually. Mostly they are in deep down of installed list and not in update list, I just have to flipping through and find them. They may be stuck in configuration status, they may also be stuck in downloading status but not proceeding any progress at all.