Depends entirely on the code base of the game.

Some games are using cross-platform engines or cross-platform libraries anyway, so it's just a compile and test operation. Some games are mostly cross-platform, but are using DirectX as the graphics API, so they'll need a wrapper to turn DirectX functions into cross-platform graphics functions - Valve's games fell into that category, with ToGL and (later) DXVK being their wrappers. Some games will depend on a bunch of Windows-only libraries, so all of those will need to be swapped out for cross-platform libraries that provide the same function - Feral's third-party ports ended up having to do this quite a bit.

How much work it is to make your code base cross-platform is entirely defined by how locked-in to a single platform you made it when you did it the first time.

Have you got your answer or do you still need help?

The amount of work required for porting old games to Linux can vary significantly based on factors such as the complexity of the original code, the technologies used, and the familiarity of the development team with Linux development. Some games may need minimal adjustments, while others might require substantial rewrites. Developers aim to retain the core gameplay and experience while ensuring the game runs smoothly and compatibly on Linux systems.

Keep in mind that the specifics can vary from game to game, and developers might use different tools and strategies to achieve successful ports.