Add notes explaining how the repo works.

.github/workflows/build.yml

@@ -47,6 +47,8 @@ jobs:
       - name: Patch config (non-PR)
         if: ${{ github.event_name != 'pull_request' }}
         run: |
+          # Replace a set of _name variables (that have # [osx] comments with
+          # an particular string (is this the certificate ID?)
           sed -i "" "s/_name: *# \[osx\]/_name: 9779L28NP8  # \[osx\]/" ${RECIPE_DIR}/construct.yaml
       - run: ./tools/run_constructor.sh
         timeout-minutes: 20
@@ -108,6 +110,7 @@ jobs:
       - uses: mamba-org/setup-micromamba@v2
         with:
           environment-file: environment.yml
+      # https://nsis.sourceforge.io/Download
       - run: conda install -c conda-forge -y "nsis==3.08"
       - run: ./tools/extract_version.sh
       - run: ./tools/run_constructor.sh
@@ -147,6 +150,7 @@ jobs:
         run: |
           sudo installer -verbose -pkg ${MNE_INSTALLER_NAME} -target / \
             || ( tail -n 30 /var/log/install.log  && exit 1 ) # display last log messages on error
+      # Check the list of packages that were actually installed by the installer.
       - run: ./tools/export_frozen_env_def.sh
       - uses: actions/upload-artifact@v4
         with:
@@ -159,6 +163,8 @@ jobs:
           qt: true
           pyvista: false
           wm: false
+      # Checking architecture is correct, files went to correct directories,
+      # checks packages via JSON file above ...
       - run: ./tools/check_installation.sh
 
   test_linux:

notes.md

@@ -0,0 +1,58 @@
+# Notes on installer mechanics
+
+Fundamental engine is <https://github.com/conda/constructor>, a tool for
+creating double-click installers from a collection of Conda packages.
+
+The README above has:
+
+> The constructor command takes an installer specification directory as its
+argument. This directory needs to contain a file construct.yaml, which
+specifies the name of the installer, the conda channels to pull packages from,
+the conda packages included in the installer, etc.
+
+In this repo, that file is `recipes/mne-python/construct.yaml`.  It specifies,
+among other things, that `conda-forge` is the only `channel` for fetching packages to provide installer.
+
+```yaml
+channels:
+  - conda-forge
+```
+
+Later, it specifies that `conda-forge` is the only channel for the provided environment:
+
+
+```yaml
+condarc:
+  channels:
+    - conda-forge
+  channel_priority: strict
+  allow_other_channels: false
+  env_prompt: "(mne-1.9.0_0) "
+```
+
+From [the contructor README](https://github.com/conda/constructor):
+
+> ## condarc
+>
+> If set, a .condarc file is written to the base environment containing the
+contents of this value. The value can either be a string (likely a multi-line
+string) or a dictionary, which will be converted to a YAML string for writing.
+Note: if this option is used, then all other options related to the
+construction of a .condarc file (write_condarc, conda_default_channels, etc.)
+are ignored.
+
+Mac (Intel, ARM) build instructions are in `.github/workflows/build.yaml`.
+
+## Build recipes
+
+These are all in `.github/workflows/build.yml`.
+
+### Mac build recipe
+
+Summary of steps:
+
+* `tools/extract_version.sh`: this is a shell script that extracts information
+  from the environment, and puts this information into environment variables.
+  Within a Github action, it also sets Github environment for subsequent steps.
+*  `tools/run_constructor.sh` runs the `constructor` executable to build the
+   package from the recipe above.