Hello.
The first thing I need to ask is; are you using a Windows PC? Because Datasmith is not supported on Mac.
Second, on the Epic Games Launcher, go to the Library tab, and under the 4.20 engine slot, click “Installed Plugins”. Confirm that Unreal Datasmith is already installed.
If it is not installed, then scroll down the Launcher page until you see the Unreal Datasmith plugin listed, and install it to the engine.
When Datasmith is installed, inside of the Editor plugin options under Unreal Studio, you should see more than just the one “Datasmith Content” option.
More details are available here: Downloading Unreal Engine Source Code | Unreal Engine Documentation