How can I keep apps open in the background?

Assuming you are using an iPhone, iOS only "runs" one app at a time.

Apps in the background are suspended. Always have been. Notifications will still come in and Background App refresh may still be running, but the app itself is suspended until it is selected again from the switch apps screen, invoked by double tapping the home button.

See >>> Switch apps on your iPhone, iPad, or iPod touch - Apple Support

Assuming you have a 6S it has 2 GB of RAM. that limits the number of apps that can be in RAM at one time; iOS uses about 500 MB (my estimate), leaving 1.5 GB for apps. That can fill up pretty fast. With modern apps that means generally less than 5 can reside in RAN, unless the most recently used apps are very small. When it runs out of RAM suspended apps are removed (as Idris points out), but they remain in the multitasking screen. A properly written app will save its volatile data to storage when it is suspended for any reason, so no data should be lost; iOS "tells" the app that it is about to be suspended so it can do this. If the app does not do this its volatile data will be lost if it is flushed from RAM.

If iOS needs the memory, it will kill suspended apps. This is not new.

It's up to the app developers to properly save the state of the app when it's suspended. Complain to the app developer(s).

My experience is that games get killed more than anything else. I think they tend to use a lot of RAM. More than with other apps, I try to make sure I leave them at a point where it won't hurt much.

Sorry, I don't use any of those so, I don't really know how they behave.

Best of luck.