What types of tools were used with traditional horse hair plaster over wood lathe How does one use these tools to make perfectly smooth walls?

I would assume a simple, but long wooden trowel similar to a mortar trowel, perhaps with some upturned skids at the end so overlapping strokes do not make unwanted marks in the plaster.

I have seen how Indonesian labourers apply cement rendering in Jakarta (live there) many many times- and they get perfectly smooth surfaces with minimal & rather primitive hand tools- so I'd assume the process is very much the same- for both internal and external walls. This has been a construction process here dating back at least 400 years according to one palace restoration foreman.

They 'throw' with moderate force large globules of the render onto the wall 20cm widths at a time, then smooth it immediately over with a mortar trowel- each throw overlapping the next. Once the labourers have roughly attached the render well, another worker with a long straight-edge plank screens over the wall to remove any bumps and detect unwanted dips.

The final labourer uses a string line attached flush with the upper-most )same plane) corner of the wall and uses this plumb line to carefully scrape off excess render as it is maybe 50%+ dry. Final touch up applications are made to the small air bubbles.

Summarily- it can be done- and superbly with simple hand tools- it just requires a lot of patience.