How to develop? Playstation/XBOX/PC/Mac/iOS/Android... How to develop?

I am sure this has been asked, but I can not find the exact answer I am looking for.

What is the DIFFERENCE in DEVELOPMENT for various systems?

I know I need licensing, that’s not what I am asking.

What I am asking, is, is it as simple as creating general content using UE4 on a Intel Based PC, and then just exporting it out to various systems? Can I just build my game, then at the end decide I don’t want it just on the PC, I want it on Playstation and XBOX, and get a license and port it, or will I have to go back in and change code and other assets?

If there are any console specific assets, can I add them to my test now, then later buy a Sony Developers License and then export out to PS? i.e. Start development now for PC, then add a few pre-made assets, like console controller code towards the end of development, or should I have everything now?

To help clarify a bit more, I will explain my situation.
My kids and I are learning game development using UE4. We are coming around nicely and want to start an “official” project to give out to their friends. No one has asked yet; but soon someone is going to ask, “Hey can I play this on my PS3/PS4/XBOX/ETC”… I don’t want to get deep into the project and then have to scrap it because it doesn’t work on something other than a PC. So, what do I need to be prepared for console/mac/tablet development, if we choose that route?

I know that to receive console code access, I will need to contact the company we wish to develop for and register for access to their SDK. Then, we will need to email consoles@unrealengine.com and let them know that we have been approved. We will send them some information, so that they can verify us & this should not change the cost of our subscription.

Developer Registration Sites

Microsoft: Developing Games | Xbox and Windows

Sony: PlayStation® Official Site: Consoles, Games, Accessories & More

UE: Other licensing options - Unreal Engine