One last question:
is there any detailed manual on how to produce gui apps using Xcode?
Do you mean with Pascal or just using Xcode in general?
If the latter, see the Xcode help or Apple's many docs and videos or docs and videos produced by many other sites.
If the former, I would suggest just following the simple example given and see how that feels to you. There's something to be said for successfully completing even a simple task with something brand new: maybe you'll see that it's not really that hard.
The main disadvantage of using Xcode with Pascal is that Xcode doesn't know much about Pascal other than syntax highlighting, so you won't have code completion or help with Pascal syntax in the Xcode code editor. That's why it might make sense to do code editing in Lazarus since you'll have those things there, and do everything else in Xcode (debugging, setting Info.plist and entitlement data, codesigning, etc.).
Pretty much everything else that you see documented about GUI design in Xcode, debugging, etc. applies to Pascal as well. After all, all Xcode is doing when it compiles is running a series of command line tools. When you use Pascal, you're just adding a couple more to the list - not too complicated, really.