Referencing MN 121, how to focus on the oneness dependent on the perception of the dimension of infinite space?

Referencing MN 121 > '...focuses on the oneness dependent on the perception of the dimension > of infinite space...' As I understand the perception of the dimension of space is the experience inside Jhana. Why did the Buddha tell Ananda to focus on oneness directly? Is this sutra meant for advanced practitioners who can focus on the infinite space? My question is how to achieve this focus?