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

I don't relate to the formless realms as being jhanic states. They are simply the opposite of the core beliefs we hold about the world. If you consider a core belief to mean a mental idea that is perpetually practiced on a daily basis such that one takes for granted that belief, then your getting closer to what the formless realms are about.

The religious overlays alongside an air of mysticism can be quite helpful for certain people with regards to entering the formless states, but let's use this 'core belief' framework to look at the formless realms and in doing so demystify them as special attainments:

The first formless realm is called infinite space. This is a direct challenge to your belief that space is finite. Have you ever looked out at the night sky and felt a poignant sense of inferiority? It might feel like you're hitting up against a mental barrier, similar to a prisoner staring longingly through the bars of their prison cell. That is a direct challenge to your belief that space is finite, for there are two nominal things: you here, and finite space there. One cannot imagine infinite space, and as such a subtle sense of sadness ensues. Focussing on the oneness of these concepts can collapse the dualistic idea of you here perceiving finite space there. The belief that space is finite came about as one grows into this world, taking on concepts & views. It is a belief that is strengthened by knowledge.

Knowledge is an important factor that defines the second formless realm, infinite consciousness, for what happens to the world when we 'know' the objects as individual perceptions? Everything is fragmented into a maddening mandala of things; that knowing becomes finite. Infinite consciousness is the collapsing of all these individual perceptions. The six sense consciousnesses are no longer anchored to a perception or a cluster of perceptions and the result is boundless consciousness and the oneness dependant upon that perception (Yes, it's still a perception, which will become clear in the next paragraph)

Moving on, the mind is further challenged... "then there must be nothingness", to which one discovers that this too is yet another perception supported by the absence of what is not there. Having challenged this belief, the mind is disillusioned and enters periods where the very fundaments of the brains perceptual structure breaks apart. This is called neither perception nor non-perception. It is here where one discovers that boundless space and boundless consciousness were also perceptions; thus the entire perceptual framework wobbles and buckles under the scrutiny of the wisdom eye.

But bare this in mind: if you have psychological and emotional hang ups, (fetters 4 & 5) this stuff will keep yanking you back into the perceptual world until you figure out how to work through them. Therefore, you will find a lot of formless realm practitioners are able to drop into the formless realms during meditation and revel about it to their peers, but in daily life they shout at their family members. There's a discontinuity between meditative awareness and real life situations brought about from the dissonance caused by the eroding away of the inner perceptual world. One should have there emotional stuff at least reasonably well put together before breaking apart your ability to perceive the conventional world, but regardless, it will be a little messy from time-to-time; nobody is perfect.

When neither perception nor non-perception stops happening, you have likely broken the 6th and 7th fetter, and one can no longer get into the formless realms simply because you've seen through those beliefs. MN 121 simply describes the progression of seeing and breaking these worldly beliefs.

The formless realms work in a kind of symbiosis to create a perceptual framework. They are simply a set of beliefs that the world gave you, and the brain gives further impetus to these beliefs, misinterpreting sensory information in a cyclic fashion, typically called samsara.

When you think of the word 'space', ask yourself what you mean by space. Visually it can only be defined by the distance between two or more objects, but we're not actually interested in a worldly interpretation; what is the perception of space? How is space 'known' in your experience?

The most standard view, I believe, would say that:

There is only this that is not emptiness, namely the oneness dependent on the perception of earth.

which precedes the passage you quoted, refers to jhāna via the earth kasina. Why the usual jhāna formula would be omitted is not clear but all the prominent teachers of jhāna that I can think of teach that you must pass through the jhānas in order and so, in this case, you would have to go through 1-4 with the earth kasina (even if extremely quickly for an advanced practitioner) and then you would be able to enter the dimension of infinite space.