Samsung
Set Default Keyboard auf Samsung
On Galaxy phones, Samsung Keyboard is set as the default input method from the first setup. Installing another keyboard app from the store does not change this automatically, because the system keeps the current default active until you switch it manually.
The setting lives inside General management, in the keyboard section that manages every installed input method. Any keyboard that is not yet enabled there stays greyed out and cannot be picked as default, even if it already appears on the home screen or app drawer.
To change it, open Settings ➔ General management ➔ Keyboard list and default ➔ Default keyboard. Enable the new keyboard in the list first if it is not already switched on, then open Default keyboard and set it as the active one.
This setting affects every app that uses text input, from messaging to browsers. If only one specific app keeps opening a different keyboard, check that app's own input settings instead, since some apps override the system default for their own text fields.