First when you want a quick analysis of a sutta, you can consult Pya Tan
In mn119, there is nothing to understand. the buddha lists various ways to "develop of mindfulness immersed in the body", including the jhanas.
To "develop of mindfulness immersed in the body", you can start with any paragraph, like this one ...
"Furthermore, when walking, the monk discerns that he is walking. When
standing, he discerns that he is standing. When sitting, he discerns
that he is sitting. When lying down, he discerns that he is lying
down. Or however his body is disposed, that is how he discerns it. And
as he remains thus heedful, ardent, and resolute, any memories and
resolves related to the household life are abandoned, and with their
abandoning his mind gathers and settles inwardly, grows unified and
centered. This is how a monk develops mindfulness immersed in the
... and you can continue to develop it, in other situations, by practicing other paragraphs like this one ...
"Furthermore, as if he were to see a corpse cast away in a charnel
ground - one day, two days, three days dead - bloated, livid, and
festering, he applies it to this very body, 'This body, too: Such is
its nature, such is its future, such its unavoidable fate'...
Some people claim that they have "mindfulness immersed in the body." because they are good at practicing one paragraph only, and are eager to say that they are good at mindfulness, instead of saying that they are good at "developing" mindfulness; but having "mindfulness immersed in the body" means precisely to be good at having "mindfulness immersed in the body" in all the situations listed by the buddha, not just one.