The component based navmesh system has been in development for quite some time, and I’ve been using it over the built in pretty much since it first came out. It has been availabe on Github since the start, but has now moved into an experimental package. You can still clone the github repo, but you may want to use the experimental package instead.
For Unity 2021 it’s quite well documented: You can just add the package by name. However, they don’t mention that you can add it in older versions. I’ve only tried this in 2020.3 but it may also work for older versions.
2020.3 does not have the “Add package by name” feature, but you can bypass this by using the “Add package from git URL”. Somple add the following when you are asked for the URL.
Hope this helps!