This is far from simple.
The following webpages may help.
https://www.extendoffice.com/documents/word/5483-word-dependent-drop-down-lists.html
https://www.msofficeforums.com/search.php?searchid=744076
https://answers.microsoft.com/en-us/search/search?SearchTerm=dependent%20drop-down&tab=&status=all&advFil=&IsSuggestedTerm=false&isFilterExpanded=undefined&CurrentScope.ForumName=msoffice&CurrentScope.Filter=msoffice_word&ContentTypeScope=&page=1&sort=Relevance&dir=Desc
In the jargon of tech writers, this seems to be called a dependent drop-down list.