https://unity.com/releases/2019-3
Those who wish to download 2019.xyz can download it from this link
https://unity3d.com/get-unity/download/archive
https://unity3d.com/get-unity/download/archive
And those who wants to download any other older version for example 2020 or early 2019 can download it from here.
When you download the version file installer, an assistant of Unity Editor will be downloaded, click on the assistant and select the modules you wish to download. Then after download is complete. You have to restart your system and then when restart is complete open Unity hub and go to installs, click in locate editor and then give path to the editor you have downloaded. All done! Now you can use any Editor version as per your requirement.