See also the schema readme and the scripts readme for this repository,
The cartography here started as a reimplementation of the raster maps at map.atownsend.org.uk, but has now evolved beyond that, including handling edge cases that the raster maps did not. The schema described here broadly matches that from here, and the display code essentially reimplements this. That is in turn based on openstreetmap-carto from circa 2014 (before cartography changes made that challenging to use in rural areas).
There has been a considerable consolidation of layers from the 76 or so in raster to 8 here.
The lua processing code that produces the data that this style processes is shared between raster and vector. The tags and values in the style .json may not even exist in OSM at all. See the schema readme for what actually gets extracted and how OSM tags get turned into the values in the .json.
Most data extracted by that schema is displayed by this style at some zoom level; an exception is boundary=administrative polygons and centroids.
The icons used to create the sprites used by this style are copied from source icons; they are shared with the svwd03 shortbread style as well. Some icons (e.g. the double-height road shields) are only used in the svwd03 style, not here.
The development of this map style here started out based on the basic Tilemaker example, and were developed using the MapLibre documentation.
The fonts used are from Klokantech, in turn based on Google's noto.
The cartography is based on this which in turn was developed from that found in OSM Carto circa 2014, based in turn on the "Mapnik style" from considerably earlier.