Since I removed the HDHomerun from Plex DVR and moved to Channels (so much better) I've missed being able to use PiP on the iPad and having a Windows Desktop window like Plex Theater provided. I've now resolved both these issues.
Apple iOS
On iOS search for and install web browser Brave. Then point a tab to your Channels server and start up a channel. Tap the playing video in the tab and you'll see several icons along the bottom, including PiP, Airplay and full screen. Tap the PiP icon and the video will now show outside of the browser window, which you can now minimise. To come out of PiP tap the PiP window and tap the icon top-right.
Windows Desktop
Do a search and install web browser Opera. Point a tab to your Channels server and start up a channel. Then right-click the playing video window and a menu will pop up. Down towards the bottom will be the option 'picure-in-picture'. Click on this option.
What you'll get is a frameless window playing the channel. You can drag this window anywhere and resize it as you wish. I often drag it to my second monitor to watch things when working.
You'll notice there are no controls on this video frame. To see those, bring back up the Opera tab and you'll notice where the video once was is now a placeholder with the words 'Playing in picture-in-picture' at the top. Hover your mouse over and you'll see the usual controls to control the channel playback. If viewing a recording you'll see a handy chapter control, allowing you to skip adverts. To send the video frame back to the Opera tab click on the icon top left.
There may be similar methods for other OS's but I don't have access to those but the above may guide you as to getting PiP using Channels and a nice Windows Desktop player.