>>2928299
there are several main techniques
usually they make many smaller pieces using with molds and/or jiggs like these>>2928629
large sections and ceilings are typically done as tiles, sculpted on a on table and then put together like puzzle and then meticulously cover up the small gaps with more plaster to make it all look seamless
then they glue them into place

but sometimes the easiest way to do it is to sculpt it directly on the wall
painting has to be done by hand with small painter brushes and sufficiently thin paint