How to add new Erlang application to a Rebar3 umbrella project?

I couldn't find it in the docs (or, more probably, I missed it), so I presume these are the steps:

1. Copy existing Erlang application (or create a new one with rebar3 new app) into <umbrella_root>/apps/ (or <umbrella_root>/libs/

2. Add the new app to the relx section in <umbrella_root>/rebar.config:

    { relx
    , [ {release
        , { your_big_project_name, "0.1.0" }
        , [ your_big_project_name_or_smth_else
          , the_newly_copied_app
        % , sasl
          ]
        }
      , {sys_config, "./config/sys.config"}
      , {vm_args, "./config/vm.args"}
      , {dev_mode, true}
      , {include_erts, false}
      , {extended_start_script, true}
      ]
    }.
   

3. Add the new app's required configuration environment variables to <umbrella_root>/config/sys.config.

4. If the new app uses a plugin, configure it in <umbrella_root>/apps/<new_app>/rebar.config.

Am I close? If yes, does that mean that umbrella applications can be nested? (This should probably be a separate question).

Yes, that should be it all you need to include an app file in a release.

Regarding nested umbrella applications, please have a look at the following thread in rebar3's site

EDIT:
The linked thread talks about having umbrella apps as dependencies, which is not supported by rebar3. Quote:

Umbrella applications of that form are just not supported as dependencies. Handling versioning and locking for a single dependencies that contains multiple apps is not a thing we ever figured out, so it's just not doable.

That does not mean that you cannot use some tricks, like using git submodules and multiple project_app_dirs configured in the root. For rebar3 those apps will be local apps, you'll need to handle them from 'outside' rebar3, though (not really 'nested' umbrella applications).