I did some experimentation recently to figure out how to make condensed milk. Having picked up the concept that it is essentially milk and sugar boiled together, I took equal amounts of both (by volume) and heated them in a pan.
Waiting until the sugar had dissolved or until the liquid started boiling yielded a thin, sweet result. Allowing the mixture to stay at a rolling boil for a little while gave some thickness but still a bit thin. It was turning the temperature down a bit so the liquid was gently bubbling and stirring until it started to become clearly viscous that gave the result I was after.
The resulting “condensate” got thicker still as it set, so it looks like it is important to stop before it reaches the final intended consistency. My next step in this experiment will be to make some condensed milk and see how it works in a recipe that calls for the stuff.