Hello, I am an android developer but I also want to learn iOS development. I thought that I should buy a second-hand laptop for learning it first then buy a new laptop. so, I want to know that is Xcode runs on older MacBook air which has 4Gb ram and 128GB from early 2014 or 2015. And I also have 1 more question which latest os will it will support how can I confirm that?
4 GB RAM and 128 GB disk space is way too limited. You need at least 16 GB RAM and 500 GB disk space available.
MacBook Air 2014 can only run Big Sur (2015 can run Monterrey as well whis is required for Xcode above 13.3).
You'll find supported OS here: https://www.macworld.com/article/673697/what-version-of-macos-can-my-mac-run.html
But if you want starting to learn Swift, not to publish on AppStore yet, you may use an older version of Xcode at the beginning to get your feet wet. And check if Xcode can install on your Mac disk space and run in just 4GB (I fear not). You will decide later if you need more.
You can download all versions of Xcode here: https://xcodereleases.com/
I also recomed you to buy m1-macbook or m1-macmini, as for me m1-macmini is the perfect decision(with at least 16gb ram and 512 gb ssd), intel-macbook is the past epoch, Apple will not support intel-macbooks in a few years.