Or, it is better to create a meta file for the library while copying the library during the installation process.

How you describe it is one way to do it, although the copied files may disappear from the package folder if the project needs to reimport the git package. The way I usually handle this is to copy the mujoco DLL straight to my Project (somewhere in the Assets folder).

To better support git based installations, maybe the install instructions could be updated with this approach?

Alternatively, maybe including the dll and meta file in the package itself could be appropriate. Or is including a dll like that not good practice?

How you describe it is one way to do it, although the copied files may disappear from the package folder if the project needs to reimport the git package. The way I usually handle this is to copy the mujoco DLL straight to my Project (somewhere in the Assets folder).

To better support git based installations, maybe the install instructions could be updated with this approach?

Alternatively, maybe including the dll and meta file in the package itself could be appropriate. Or is including a dll like that not good practice?

Including a dll file in the repo doesn't seem to be a very well practise, although it is convenient for users to import. Doing so will lead to the increment of the repo's size at every update, as the dll is a binary file.

As indicated in the document, "On Unity version 2020.2 and later, the Package Manager will look for the native library file and copy it to the package directory when the package is imported." But the Package Manager will only copy the library file itself, not creating the .meta file at the same time. I think this will be the major problem.

P.S. I use Unity 2020.3.48f1c1. I'm not sure whether on higher version of Unity this issue will occur or not. I imported the package using "Add Package from Git URL" command in the Package Manager and entered the URL, https://github.com/google-deepmind/mujoco.git?path=/unity#3.0.0

Therefor, the install instructions should be updated to indicate this.

I already open a pr to fix the problem.