How to port a 3DS game

You rebuild it. Confirm you hold the rights, inventory your design documents, art sources, audio, and any source code, pick a target platform, rebuild the core gameplay loop in a modern engine, redesign the two screen layout and the input, test on the target hardware, and export.

It depends on how much content there is and how much of the original material survives. A small game with intact art sources and design documents is a short project. A large game with missing source, licensed assets, and heavy stylus mechanics is a long one. The content volume and the input redesign drive the schedule more than the code.

Logic yes, platform calls no. Systems like combat rules, progression, and state machines are still valid as reference and often translate quickly. Anything that talked to 3DS hardware, the dual screen output, the touch layer, the stereoscopic depth, save data, or Nintendo online services has to be rewritten.

Two things. Deciding what happens to the bottom screen, which is a design problem rather than a technical one. And converting stylus mechanics into controller or mouse input without losing what made them work. Resolution and performance are comparatively easy, since modern hardware far exceeds a 3DS.

No. Solo ports are common, especially with an engine that can generate playable scenes from a description and let you refine them in plain language. A team helps most with content volume and platform certification work, not with the rebuild decisions.

No. Buying a copy gives you the right to play it, not to rebuild, publish, or sell it. Port a game you made or one you have licensed. This process does not involve dumping cartridges or redistributing other people's games.