Can’t switch between apps on iPhone.

Please check to make sure that you are doing the Forced Restart correctly. Most users have no trouble briefly pushing the UP Button, then briefly pushing the DOWN Button.

But, some users do not hold down the SIDE Button long enough for the Apple logo to appear on the screen.

Instead, they release the SIDE button when they see the option to Slide to Turn Off. You have to ignore the Slide to Turn Off screen and keep holding the SIDE Button down for another 15-20 seconds or more until the Apple logo appears. Then, you can release the SIDE button.

There are two possible results with the Forced Restart:

1) The phone returns to normal operation

2) The phone has a hardware problem that will need attention at the Apple Store or authorized Apple Service shop

If the forced restart is not working it sounds as as hardware issue to me.

Please update your iPhone to the lastes iOS version and if the problem persists ,Contact - Official Apple Support

to set up service for your device

What is happening when you swipe up from the bottom of the screen? Are you swiping up to about the middle of the screen?