Please note that it will take you probably several hours to setup everything correctly, if you want to develop for mobile. Here you will find many links to other places.
You should always use latest Xcode. Officially Qt5.15 only supports Xcode 11, but personal experience has shown that both Xcode 12 and 13 work without problems, so you can ignore the warnings that will be shown. For iOS 15 you need Xcode 13. If you want a specific version, see: Xcode Releases. Prepare for a 10 GB download. You'll need a (free) Apple ID, which is required anyway for deployment to devices.
For mobile we don't want latest Qt6, which currently still misses important QML modules. There is basically no way around the online installer, because you don't want to build the cross-compiled part of Qt for yourself.
See reademe-qt for the Qt online installer. You'll need to register with an email.
Make sure to select the following in the online installer:
You really want the online installer. Yes it's a 1 GB download, but it brings
all the examples, demos, tutorials, documentation, Qt Designer, all QML
modules, so you won't miss anything. And you can easily add/remove parts later
(see ~/Qt/MaintainanceTool after installation), or install Qt6 in the future
(side by side).
Next step is cross-compiling ECL. See readme-prepare-ios.
Now build both the lqml executable and library, see
readme-build.
You may now try to run a desktop example, to see if it works. Every example has a readme which explains everything.
The best example to take as a template for development on mobile is examples/advanced-qml-auto-reload.
Please see readme and readme-build of that example.
Since adding an app icon requires an asset catalog with many different sizes, it's convenient to automate this process. In the App Store you find 'Asset Catalog Creator', which is free for basic use like creating app icons.