Can conda channels include multiple channels?

I'm reading the documentation on Conda channels, and the documentation appears to state that:

• By default, Conda uses the defaults channel. This can be verified locally:

% conda config --show channels
channels:
  - defaults

• The defaults channel is hosted on repo.anaconda.com

However, a quick glance at the landing page shows that this channel seems to include multiple channels, such as pkgs/main, pkgs/free, etc. This is extra confusing because this reflects the output of a similar command to list default channels:

% conda config --show default_channels
default_channels:
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r

So my question is, what exactly is a channel? Is defaults just a special, hard-coded alias for a list of default channels, namely pkgs/main and pkgs/r? Or can any channel declare that it includes multiple channels within itself?

In your .condarc file there are several keys of interest: channels, custom_multichannels, default_channels, and channel_alias

Any values listed under channels are a single channel or a multi-channel that conda will search. A single channel such as conda-forge can be listed using just the channel name or the url.

channels:
  - https://my.customrepo.org/dev/win-64
  - conda-forge

But you can also define multi-channels, which are just a collection of channels with a name, and then reference that. In the example below, the internal-dev is not a channel on http://conda.anaconda.org, but instead is the name of a multichannel pointing to a set of internal channels.

channels:
  - conda-forge

custom_multichannels:
  internal-dev:
    - https://conda.mywork.site/dev/noarch
    - https://conda.mywork.site/dev/win-64

Now, you can refer to the interanl-dev channel when using conda:

conda create -n testenv python some_dev_package -c internal-dev

You can also reference it in your list of default channels in the .condarc file.

channels:
  - internal-dev
  - conda-forge

custom_multichannels:
  internal-dev:
    - https://conda.mywork.site/dev/noarch
    - https://conda.mywork.site/dev/win-64

The default_channels key is just another multichannel, with a pre-defined name of defaults. The following two .condarc YAML files are functionally equivalent:

channels:
  - internal-dev
  - defaults
  - fastai

default_channels:
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r

custom_multichannels:
  internal-dev:
    - https://conda.mywork.site/dev/noarch
    - https://conda.mywork.site/dev/win-64

channels:
  - internal-dev
  - defaults
  - fastai

custom_multichannels:
  defaults:
    - https://repo.anaconda.com/pkgs/main
    - https://repo.anaconda.com/pkgs/r
  internal-dev:
    - https://conda.mywork.site/dev/noarch
    - https://conda.mywork.site/dev/win-64

The last key to be aware of is channel_alias. This sets the default server to use as the root for finding channels. The default is https://conda.anaconda.org. However, if you have an internal conda server that you use almost exclusively, you can change this value so you can reference the channels by just their name.

channel_alias: https://conda.mywork.site

channels:
  - internal-dev
  - defaults
  - fastai

default_channels:
  - anaconda
  - r

custom_multichannels:
  internal-dev:
    - dev-build
    - dev-test

All of the above channels point to https://conda.mywork.site/<channel> and not to an outside server. To point to a channel on the internet, you would need to use the full URL (i.e. https://conda.anacond.org/conda-forge)