What are extension repositories?

Extension repositories are where you can get third-party extension files (.apk) for Tachimanga.
The URL for an extension repository usually ends with index.min.json.
Here's an example of what the file looks like:

[
  {
    "name": "Tachiyomi: Komga",
    "pkg": "eu.kanade.tachiyomi.extension.all.komga",
    "apk": "tachiyomi-all.komga-v1.4.50.apk",
    "lang": "all",
    "code": 50,
    "version": "1.4.50",
    "sources": [
      {
        "name": "Komga",
        "lang": "all",
        "id": "4508733312114627536",
        "baseUrl": "",
        "versionId": 1
      }
    ]
  },
  ...
]