current unity version
2020.3.11LTS Long Term Support Release
reason
Searching directly in the package manager cannot be found, even if the preview package is opened.
The reason is that the official closed the access channel of the preview package in the new unity version.
You can take a look at the official blog explanation - https://blog.unity.com/technology/package-manager-updates-in-unity-2020-1
git download
But you can download it through git and
enter com.unity.entitiesit and it will be installed automatically.
At the same time, entities will depend on many other packages, which will also be downloaded together.
Official api documentation
The current version is 0.17.0, and the api of different versions has been greatly updated, so you need to check the official api documentation .
Reference:
How to install the Entities package of DOTS in Unity2020.1?