The Hybrid Placeholder is now also available as a NuGet package. It's an alternative to the Sitecore package. If you want to try out the Hybrid Placeholder for the first time the Sitecore package is still easier to use. You can follow the instructions in this video if you want to use the Sitecore package: https://youtu.be/-8k7jzuRk9U
If you want to implement the Hybrid Placeholder you need to inherit from the HybridRenderingContentsResolver. If you install the NuGet package that class becomes available in your project. Previously you needed to reference the DLL from the installed Sitecore package.
The NuGet package has a dependency on Sitecore.JavaScriptServices.ViewEngine 15.0.0 which is on MyGet. So make sure that you have the following package source in your project: <add key="Sitecore NuGet Packages v3 feed" value="https://sitecore.myget.org/F/sc-packages/api/v3/index.json" />
You can run the following command:
Install-Package HybridPlaceholder
Or view the package here: