how to find and install
How can I install the full Xcode for High Sierra (10.12.6)
I'm not sure what you mean by the full.
But you can download the latest version of Xcode from the More Downloads page.
You should better check which version is the latest for High Sierra, visiting Xcode wiki.
(Seems 10.1 is the latest for High Sierra. You may need to upgrade your macOS to 10.13.6 if not yet.)
Please remember, you need to use Xcode 11 (comes with iOS SDK 13) or later to build an iOS app for App Store.
Meaning you need a Mac capable of running newer macOS to build apps for App Store.
But you can download the latest version of Xcode from the More Downloads page.
You should better check which version is the latest for High Sierra, visiting Xcode wiki.
(Seems 10.1 is the latest for High Sierra. You may need to upgrade your macOS to 10.13.6 if not yet.)
Please remember, you need to use Xcode 11 (comes with iOS SDK 13) or later to build an iOS app for App Store.
Meaning you need a Mac capable of running newer macOS to build apps for App Store.