[1] suggests it uses a Constraint Satisfaction Problem solver and according to the network tab of my browser the solver seems to run on the server (the data is exchanged over a WebSocket connection). I'm wondering what kind of algorithm the solver uses too, and if it relies on any heuristics or specific data structures for this problem. For example [2] shows best results using bitarrays and a mix of "forward checking", "conflict directed backjumping" and "dynamic variable ordering". (which are fancy terms to describe the methods one usually comes up with to solve a Sudoku grid)
https://www.youtube.com/watch?v=6cpTEPT5i0A&list=PL3C690048E... (note: this is the first video of a playlist)
https://www.dimensions-math.org/Dim_E.htm
https://en.wikipedia.org/wiki/Dimensions_(animation)