My starting point for a new installation of openwrt has always been the sysupgrade image. I'd like to use the "customize installed packages option" aka image builder. It looks to me like the dropdown list contains the packages in the "kernel" image.
I'd like to know what packages are added for the sysupgrade image.
When using the image builder does it add the dependencies for a package, or do I need to be added as well?
The default packages for your device, nothing else. Those packages are automatically selected when you select the device for the build. You just need to select the device and the packages and their dependencies are pulled in automatically.
(For release branch builds also the LuCI GUI and its dependencies are included. For the master development branch you need to select LuCI by yourself.)