In South Asia during the Lord Buddha’s time much of the land was covered in forests and groves and this was where the wandering mendicants of the different orders would pursue their religious practices.
The Lord Buddha spoke of the “foot of a tree” as the basic shelter for bhikkhus, and this is usually still affirmed to every newly ordained bhikkhu.
Later, monasteries were established and well-endowed, and the focus shifted to a more settled life.
Mostly only the “forest monks” now live in the forest where it is quiet and conducive to meditation.
Many more monks will live in the village monastery or go to a monastery in the town to study the scriptures.
The Lord Buddha said this about the basics of shelter, whether in the forest or city:
Properly considering the lodging, I use it: simply to ward off cold, to ward off heat, to ward off the touch of flies, mosquitoes, wind, sun and reptiles; simply for protection from bad weather and for the enjoyment of seclusion.
Learn more about the Theravadin Buddhist Monk’s Rules here:
The Bhikkhus’ Rules – Guide for Laypeople