A mod and a port are very different.

So ports are taking a game developer for one platform and getting it to work on another. This can be complex due to different system requirements not just technical specs but take porting from PC to PlayStation. A lot of PC games don't need to support a controller but porting to PlayStation that's your only player input. Additionally you have different screen requirements like a UI that's usable from 20 feet away on a 30 inch TV. Letting the player adjust the UI scale. Of course, there is also the more technical side. Like the PlayStation uses different floating point algorithms than Windows and thus I've seen something like jump pads launch you double the height compared to PlayStation. You then have to correct the code and make it cross platform. You could do this by changing the order you do operations or simply in My example divide by 2 on Windows (not recommended).

A mod is taking a game and using it as a base to make changes and add to the game or make a new one. Some games put specific APIs in their game to support modding. Typically by using a scripting language which can be either replaced or added to a specific folder. Other times people forcibly make mods by opening packed files like jar files or pak files and replacing their files or adding new ones. These two methods could also be used in combination. The benefit of scripting languages are that they allow cross platform mods fairly easily since the scripting language interpretor is built into the game.

I've been a game developer for 10 years and have done both porting and making a game moddable.