Word is a complex program despite Microsoft trying to make it easy. They make it
look easy.
Spend some time learning to use it, just as you would take time to learn how to use any other new tool.
Check your Word advanced options. Do you have either of these boxes checked?
Those control the feature you are discussing and should both be unchecked.
Unfortunately, because Word is so complex a real user's manual would take up a bookshelf, and not a small one. Instead MS relies on its help system. That, in turn, relies on the user knowing the jargon.
When I type "typing replaces text" in Help, I do get help, but it is not all that obvious.