Short answers, pulled from the story.
James Watt conceived a radical idea with a separate condensation chamber in 1765 while working on his own design. This innovation allowed steam to flow into a secondary chamber that remained cold at all times below atmospheric pressure. The process turned water vapour into liquid water inside the condenser and eliminated heat loss from the working cylinder.
The new design used about half as much coal to produce the same amount of power as previous models. This efficiency gain occurred because cold water was injected only into the secondary condenser rather than the main cylinder. Surrounding cold water maintained a temperature near freezing and equivalent pressure levels throughout operation.
Watt introduced a system using sun and planet gears in 1781 to convert linear motion into rotary motion. This innovation made the engine useful beyond its original pumping role as a direct replacement for water wheels. Power sources could now be located anywhere instead of needing suitable topography or water flow.
Three engines were ordered in 1777 for Wheal Busy Ting Tang and Chacewater mines. Most customers replaced Newcomen engines where fuel was expensive particularly Cornwall. Engines became attractive where fuel prices justified installation despite high initial costs.
Old Bess built in 1777 stands as oldest surviving Watt engine now housed in Science Museum London. An 1812 Boulton and Watt engine at Crofton Pumping Station Wiltshire still performs original function pumping water for Kennet and Avon Canal. A 1788 Boulton-Watt engine resides in Science Museum London alongside an 1817 blowing engine decorating Dartmouth Circus traffic island Birmingham.