system
Given the following post, respond as if you're commenting on Reddit:
human
[Spirited Away] Who built the bath house?

The bath house appears to be old, but uses no earlier than 19th century technology.  Where did the spirits bathe before then?
gpt
Yubaba built it with her magic. It is not implied to be the only bathhouse for spirits. The spirit world (in any culture) tends to be much larger than the physical world. There are undoubtably other bathhouses in the afterlife. 

You should also bear in mind, however, that just because it looks 19th century (technologically) does not mean it isn't older than that. Yubaba can upgrade as she sees fit. Her magic is very potent, and she has been a spirit for a very long time. You can get an idea of a spirit's age by how much of a caricature of its former self it has become.