Executing
$InstallAdditionalPlugins=True; Import["https://wolfr.am/NHGLnZMa"]
from a Mathematica 12.1 Notebook on macOS or Linux or Windows or
wolframscript -code '$InstallAdditionalPlugins=True; Import["https://wolfr.am/NHGLnZMa"]'
from Wolfram Engine
automatically downloads Eclipse2020-06, installs it to a folder Eclipse2020-06 (.app on macOS)
and downloads, installs and configures Wolfram Workbench, an IDE plugin developed by Wolfram Research.
Omitting $InstallAdditionalPlugins=True; does not install the additional plugins listed in $AdditionalPlugs.
What happens exactly is commented in the code.