There is not that much to it. You can link data blocks to one file from another file using the 'Link...' function in the File menu:
You select another file and then just choose what you wish to link to your file:
As you can see that may be a lot of different things, like objects, or object data, materials and so on.
Once you link something from another file, you can only edit it in that file, unless you break the link for it by making it local:
Once you edit something in the file it is linked from, you need to reload it in the file it is linked to. That can be done by saving and reloading the file.
If you work alone on small or medium sized projects this might not make as much sense as when working in a team of people on huge projects. You can have a library of assets that update in all the files of the project that they are linked to once they are updated in their files without any extra work. Obviously, if you have only one or 2 main files in a project, this is not that useful.