Hey @Deal,
Are you doing your changes within the extensions/defaultExt folder?
The yarn run build-dev:defaultExt --watch will only watch the extensions/defaultExt folder (see the angular.json config).
The footer.component.html is a core file, you need to do watch on the core and shell.
Just a warning: the changes done to core folder are not upgrade safe. Its not recommended. All files there will be replaced whenever you upgrade. You can track the changes by using git or similar, but is going to be hard work.
There is a guide on building the the front end on: