Host Multiple Domains In One Monorepo On Netlify

TL;DR

Go through the standard process but use the GUI to set the "Basic build setting -> Base directory" to point to the individual site roots. (You can also use the CLI, but I don't know how to do that yet)

I'm building an example set of domains for a blog post. One domain for each letter of the alphabet. While I could make individual repos for each domain that would make a noisy mess in my github account. Instead, I'm putting the source for every domain in a sub-directory of a single repo and deploying from there.

For this example, I'm using a single vanilla HTML page for each site. The directory tree looks like this:

.
├── letter_sites
│   ├── a
│   │   └── index.html
│   ├── b
│   │   └── index.html
│   ├── c
│   │   └── index.html
│   ├── d
│   │   └── index.html
│   └── e
│       └── index.html
└── main_site
    └── index.html

To deploy the main site, I setup a New Site in Netlify, pointed it at the repo and then manually set "Basic build setting -> Base directory" to "main_site". Doing that auto populated the "Publish directory" with "main_site/"

when the publish was done, I set the domain, the enforced https.