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I assumed that the flour was the food
This is correct. There are carbs available in the flour that the yeasts will metabolise, but there are also microbes* will occur naturally on the grains and therefore will make their way into the flour.
and that leaving the vessel open for a period of time was to encourage wild yeast spores to settle onto the starter.
Microbes need oxygen so never seal your vessel unless you want an explosion (guessing you probably knew that anyway). I ferment a few things at home and normally cover jars with a tea towel which allows free circulation of gases and microbes but keeps insects out.
*I use the term to include yeasts but also [not necessarily bad] bacteria
Arducius
I assumed that the flour was the food and that leaving the vessel open for a period of time was to encourage wild yeast spores to settle onto the starter.
Although that does sound like a fairytale