Sacred Geometry is an ancient Greek concept that has nothing to do with Freemasons. Pythagoras was the most famous person to push it. It's that thing where they thought the world was made out of the Platonic solids, tried to explain the planetary orbits in terms of polygons, et cetera. It remained huge through Greece and Italy until the 1400s. The Leonardo piece "Vitruvian Man," where the guy has his arms at a couple of angles, and all the describing shapes behind him? That's Sacred Geometry. People used to think those shapes were evidence of design by God.
This was extremely common throughout Europe, especially in Christian churches and cathedrals (much like Islamic) until the late 1400s, when an Italian named Alberti wrote a book that basically pushed religious architecture forward 100 years in ability to make safe large buildings.
That book pushed for a return to circles and the cross, and most of Europe followed. However, that was also 60 years before the Anglican split, and about how long it took things to move from one end of a continent to another back then, so when it was getting to England, England decided to say that the rest of Europe was losing the beat, and stuck to the more complex Greek styled visuals
Most of Europe removed the Sacred Geometry, but England (and Italy) kept theirs. England in particular sees this as a tie to historic legitimacy; one of the things that makes Their Church Different And Better (tm). "We didn't forget the old ways" kind of thing.
Freemasonry? This isn't the Flintstones.
Anyplace to read up on it? Obviously their ceremonies are full of masonic and other esoteric pagan ritual. THey are a pagan Germanic-druid hybrid after-all.
Freemasonry isn't pagan in any major sense. It's pretty solidly Abrahamic, and parts of it are explicitly Christian. The closest you'll get to paganism is a continuation of themes and symbolism which are shared in common between ancient cultures.
https://www.ugle.org.uk/about-us/whos-who/12-governance/155-...