Include icon in manually created .app bundle
I believe you can change the icon by simply dragging and dropping the icns file onto the icon displayed in the upper left corner of the Get Info window. For example, if I right click on the test.app shown in the Finder on my Mac and select Get Info, I will get the following popup window.



Below is a folder with some icns files.



Dragging the os_centos.icns icon and dropping over the icon shown in the upper left corner of the Get Info window, will produce the following change.



You can convert files to .icns by visiting the website https://iconverticons.com/online/. Online conversion is free.