If I am packaging software for gentoo, all I have to do is translate the build instructions from the project’s documentation to gentoo’s package recipe.

It's the same for Nixpkgs.

In nix, it seems that it is not that simple and you’ll have to do some exploration. Am I wrong?

In well behaved build systems, it's likely easier to package than most other distros. If it's not as well behaved you will have to do some "exploration" and the complexity can get quite out of control if the build system is exceptionally terrible.

Here is the package for the GNU hello program which uses a well-behaved build system:

https://github.com/NixOS/nixpkgs/blob/94b11073db6a7ca5733bc2d45378d800d9542975/pkgs/by-name/he/hello/package.nix

If you ignore the optional passthru.tests, this is very simple. You provide metadata, sources etc. to the generic mkDerivation function and that's it. The most complex non-standard thing this derivation does is enable the build system's tests.

You don't even need to run the provided build instructions because Nixpkgs' stdenv abstracts those away. If it finds a makefile, it'll automatically run make and make install with the correct flags for instance. Same for other standard build systems; if you pass cmake into nativeBuildInputs, it'll attempt to build, install, check etc. using cmake's standardised interfaces.

If the build system is poorly behaved however (like for instance Anki's), you will have to get into the weeds and do some rather advanced things:

https://github.com/NixOS/nixpkgs/blob/94b11073db6a7ca5733bc2d45378d800d9542975/pkgs/games/anki/default.nix

Luckily though, most packages aren't like this.