On iTunes I've uploaded my apps for iPhone iPad users, on the side menu on the App Store menu on iTunes, I see a blue link I can click - "Add macOS App", if I click it will it make the app I have available for iPhone
No, its just about having one listing, in the old days you could only have separate ones. You still have two different apps.
If so, then isn't it the easiest way to get my app to the Mac Store?
Correct, you have to build a dmg and go through a series of steps to meet the mac app store spec.
Will I lose any functionality? like for instance, the app I've created for iPhone is supporting keyboard input, which doesn't come handy with an iPhone device that is using only a soft keyboard, but if now it goes on a Mac device I would like it to have support for keyboard input, will it have this functionality?
As said, the ipa will be from a separate entity to the dmg.
That said, its different on M1 macs, because they can use iOS apps, in which case the mac maps user interaction to iOS specific behavior. In the iOS config you can set if you want to support M1 macs.