This post will explain you "showing angular material modal popup at the center of the screen".
When you are working with "Angular Material" modal popups, sometimes you might observe your modal popup not at the center.
At the time you need to check your css files are properly included in "angular.json" file.
If you not find below css file in "angular.json" file , then you may not get modal popup at the center of the screen.
"styles": [ "src/styles.css", "node_modules/@angular/material/prebuilt-themes/indigo-pink.css", ],
Try to add the above css file and it works.