Depends on the game and the extent of mod.

For doom its important to understand the difference between port and mod. Doom has 3 components, the physics engine that is the backbone skeleton of all the games internal code for movement bullet firing and all the functionality. 'Assets' which are the skin of the game, sprite artwork for enemies, walls, skyboxes, entities, pickups, GUI, sound and music. Maps are the flesh and blood, and using a map maker like SLADE can let you make your own a 'mod' can be as simple as a retextured sprite or as extensive as a full asset flip with new sounds and everything. Or a new set of maps.

Most mods don't touch the physics engine though unless its an entirely new game built off dooms engine such as heretic.

Dooms physics game engine was open sourced a very long time ago meaning that the skeleton is free and open source. someone can make a 1 to 1 functional clone of doom if you make your own assets and maps (that's the basis of the Freedoom IWAD project).

Ports are rewrites of the physics game engine either from its original source or completely new. Either to get the game on a console or new computer architecture. The original doom was for DOS, and when everyone moved to windows 95 native DOS comparability was lost and fans had to use inefficent emulation through Dosbox. Thats why source ports exist so that the game engine can run on newer computer hardware natively.Blood is a infamous case where the source code of the original game was lost, the new remaster of it uses a completely different engine and thus its gameplay cannot be 1 to 1 faithful.

Some games are made with modding in mind. Minetest comes to mind as its both a game engine and a playable game meant for users to build their own games. Surge the Rabbit is in the same vein being a 2d sonic clone/platformer engine meant to me nodded.

Some games are a nightmare to mod, Minecraft java edition for instance. Minecraft mods require mod loaders which inefficently tear apart the games code and patch in the mod content in a Frankensteins monster of a game code. Microjang at one point point promised modders an official supported game API but gave them the finger after they realized bedrock edition was where the money is.

Difficulty ranges on a scale then I guess. Easiest are very old games with open source ports that just require some basic image sprite and midi changes. Usually map making is very easy too. The medium is modern open source game engines that are intended for modding in mind. The hardest are locked down AAA games where the devs either ignore the modding community or even actively harm it.