By design, it should not be necessary to manually close any open Apps. iOS/iPadOS should transparently and seamlessly manage its available resources as you switch between Apps and tasks. Force-closing and reopening an App is less power efficient - as re-initialising an App requires significantly more battery power than resuming an App from its dormant state.
While it is generally unnecessary to do so, Apps can be force-closed, if necessary, using the App Switcher...
To expose the App Switcher page, swipe-upwards from the bottom edge to the centre of the screen, pausing briefly before lifting your finger from the screen; you will see a thumbnail image for each of your currently open Apps. Apps can be force-closed by swiping the corresponding thumbnail off the top of the screen.