After downloading a new virtual keyboard, such as Microsoft SwiftKey, the keyboard must first be enabled or added in order to select it as the default keyboard.

Note: The various keyboards can be downloaded and installed from the Google Play Store.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  System
Step 3:  Tap on  Languages & input
Step 4:  Tap on  Virtual keyboard
Step 5:  Tap on  Manage keyboards
Step 6:  Enable an installed  keyboard
Step 7:  Confirm with  Turn on
  1. Open the Settings
  2. Tap on System
  3. Tap on Languages & input
  4. Tap on Virtual keyboard
  5. Tap on Manage keyboards
  6. Enable an installed keyboard
  7. Confirm with Turn on

OnePlus Instructions