Samsung optimalization

Some methods to reduce the impact of the system constantly killing every app after you leave it:

Keep Open: 

Up to 3 apps can be saved with the Keep Open menu option found by clicking the icon above the app you want to keep open in the multitasking view, as noted by others. For these three apps, the problem is more or less completely solved.

Disable battery restrictions:

Background usage limits: Go to Battery > Device usage limits. Turn off Put unused apps to sleep. Under Never auto sleeping apps add every app you want to keep alive.

Device Care: On the Device Care screen:

Performance Profile: Set the Performance Profile to Standard

Auto Optimization: Turn off all options under Auto Optimization

Customization Service: Open the 3 dots menu in the top right and go to Settings, turn off Customization Service

Unrestricted Battery Usage: Go to each app's App Info page for the apps you want to stay alive. Make sure Remove permissions if app is unused is off, go to Battery and set to Unrestricted.

Adaptive Power Saving: On the Battery screen click on Power Saving, click on the 3 dot menu in the top right and select Adaptive power saving. Turn it off.

Excluded Memory Apps: Go to Device Care > Memory > Excluded Apps and add every app you want to allow to be cached. This isn't super reliable, but some apps will benefit.