Hey YuriNikko,
We understand that you're having an issue with random screenshots since installing iOS 14, and we want to help. With iOS 14 a new feature was introduced called, Back Tap. With this feature, you can either double or triple tap the back of your iPhone to have it complete the desired action. Screenshot is one of the choices. To see if this is turned on, tap Settings > Accessibility > Touch, then scroll to the bottom for the Back Tap settings. Here's a bit more information on that feature: Adjust touch settings on iPhone
If this setting isn't turned on, do you use a case with your iPhone? If so, it could be pressing the buttons, does removing it seem to help?
Keep us posted.
Take care.