This repo merges some mock "dynamically" fetched content via src/_data/posts.js, and merges it with some posts in the src/posts/*.njk directory and puts them all into a single collections.notes collection.

See the objectively boring src/posts/pagination.njk script, as well as it's front matter in the src/posts/pagination.11tydata.js template data file (since it uses some JavaScript for eleventyComputed values, and I prefer it to JavaScript front matter to shim the properties from the JSON data file into computed Eleventy properties).

Also note the src/posts/posts.11tydata.json directory data file, which sets a default layout and tags for each post in the ./src/posts/* directory.

Finally, there is a src/posts/tags.njk file which paginates over the collections.notesTags collection (see .eleventy.js) and creates tag archive pages for the posts that are in the "notes" collection.

Most interesting is the questionable use of collection hacking in .eleventy.js:

eleventyConfig.addCollection("dynamic", collectionApi => collectionApi);

When using Nunjucks syntax, this lets us do scary things like this gem in src/posts/tags.njk:

{# This weird live querying of collections only seems to work w/ Nunjucks syntax. #}
{%- set posts = collections.dynamic.getFilteredByTags("notes", tag) -%}

Is it a good idea? Probably not.
Did it work during my 3 minutes of testing? Sure.
Was it easier than re-building collections for every dynamically set tag? Ab-so-lutely.
Was it a good idea? Probably not.