I hope you remember the spoon scene in The Matrix. — Copernicus

A crap film.

Exactly. — Copernicus

It's a performative contradiction.

How am I related to the chicken? — Copernicus

Ah! There's the proof! He denies it again!

Even if - and I want to make this perfectly clear - even if there is no obvious relation between you and the chook, that does not say that there is no relation.

Just as you say all our deeds are selfish.

there is no relation. — Banno

If you meant from the aspect of causality (butterfly effect), then sure, we're related. But if you meant uniformity like electrons, then you're missing the point.

performative contradiction. — Banno

It's not.

"I'm sure everything is unsure" = Everything is unsure.
"I'm unsure if everything is unsure" = Everything is unsure.

@Copernicus

Does the relationship between whatever a "self" ought to reference and the "body" (of that self) have any relevance to the opinion that "all acts are selfish acts"?

There is sometimes the anomaly of conjoined twins who share a composite body, like Abby and Brittany Hensel. Maybe we would say two selves (individuals with distinct personalities, minds) share one body, while one controls left side and the right side.

In accordance with your OP, they both are always acting in their individual self interest all the time. Mutual coordination/agreements, trade-offs for the shared self (as shared body), is just a "refinement of selfishness". In one sense they share a self and another they don't.

This same kind of relationship extends to people who do not share a body in the conventional sense, but are embedded in ecological environs/processes (dependent functional relationships between each other that comprise "organizations").

How do we assign what belongs to self (as body, or otherwise) and what doesn't in terms what goes into instantiating whatever the "self" is?

Someone could do an act and not really know what caused them to act. They may form a rationalization/narrative that explains their action, but the motivation could actually be caused by something completely unknown to them. Suppose a benign brain tumor enhances the reward for gambling in a person towards self-destruction. For the person the desire/reward of gambling might align with their desire/will at one point in time and be against their desire/will at another. They know with respect to a future condition they'd be better off if they could inhibit their impulse yet they always succumb to the act.

What goes into instantiating the self goes beyond the limited awareness of any self.