OnePlus
Set Default Keyboard auf OnePlus
Installing a keyboard app from the Google Play Store does not automatically make it active. Android keeps every installed keyboard disabled until you enable it and pick it as the default input method. That is why the stock keyboard keeps appearing even after a new app is installed.
On OnePlus phones running Oxygenos, this switch happens in the input settings, not inside the keyboard app itself. Open Settings ➔ System ➔ Languages & input ➔ Virtual keyboard ➔ Default keyboard, then choose the keyboard you want to use from the list. Only keyboards you already enabled under Virtual keyboard show up here.
If your preferred keyboard is missing from the list, it likely still needs to be turned on first before it can be selected as default. Once set, the new keyboard opens for every text field system-wide, including messaging apps, browsers and forms.
Switching the default keyboard does not remove the old one. The stock keyboard stays installed and can be selected again the same way at any time.