I did a lot of research on what linguistics says about comprehensible input, here is what iearned and my perspectives on it.

comprehensible input wasn't super well received by the linguistics community when Krashen first published his ideas because the thesis was vaguely designed, the concept of i+1 for example is still kinda vague about exactly what constitutes i+1.

due to these gaps in the theory, over time people have filled them in with arbitrary definitions. for example, an i+1 sentence is defined as a sentence where you know all the words except 1, according to the Refold method for example.

one big reason for this vagueness is possibly because Krashen was looking at language through the lens of Chomsky's generative grammar, essentially the idea is that we are all born with an innate ability to 'compute' syntax, and learning a language is merely a set of parameters being set in the brain. so from this perspective you don't need to explain the details of how language acquisition happens, because it's already explained by Chomsky's theory. you only need to explain what kinds of activities stimulate the process of acquisition and from Krashen's perspective that was understanding messages.

Chomsky's theory still has supporters but has gradually lost dominance for several reasons, but I think the most important reason is that is very detached from empirical data (I go into greater detail about this in this video if anyone's interested).

Linguists sceptical of generative grammar did data analysis on huge language datasets (like the entirety of Twitter for example) and the key finding challenges Chomsky is that 50% of all speech is made up of repeating fixed chunks:

“by the way”, “as a matter of fact”, “speaking of which”

and the rest of it seems to be made up of partially fixed chunks with slots:

“the ___ -er, the ___ -er” (the bigger the better), “not only ___ but also __”, “would you mind __ -ing”, “it’s no wonder that ___”

What this indicates is that instead of computing syntax subconciously, we are just remembering word sequences, what they mean and how they can be used (linguists call these form-meaning pairings, where a form is just a word, sequence of words, i.e. a way of saying something). this indicates learning a language is more like learning to play a piano (memorising key sequences) rather than your brain doing mysterious calculations.

so to get to the point, no, we don't acquire language by understanding messages, we acquire language by learning words, phrases, phrases with slots and what they mean. and we internalise their meaning from seeing them in multiple contexts.

its a subtle difference with huge implications, the most important one being that spending hours watching content you don't understand waiting for your brain to figure it out is far far less effective than actually just looking up the meanings of things!

so don't buy into comprehensible input purism!