While MacOS has had programs that allow regular users to create custom keyboard layouts (such as Ukelele) for a long time, the same is not true for iOS. Users still need to rely on developers produced by developers and offered through the app store. Below you will find some information on how these can be done by people with the necessary skills. The first link is to create files for the Keyman app, which must then be compiled on a Windows machine. The other 3 are training programs for IOS programmers.
https://www.raywenderlich.com/49-custom-keyboard-extensions -Komme start