I'm not able to use my headphones (port busted) but it still got me spooked. Since I haven't seen anyone talking about the possible story of the game I will!
So based on the fact that the man outside is attracted to your phone according to your friend, it probably uses electronics to track it's prey. It seems to "infect" if that's the right word the electronics and that shows how it's feeling?
Your friend also seems to be similar in that regard? But not a monster, since they remember everything that happens and can also use your phone to convey messages, they're clearly not human, but not evil either.
Now onto the glitches and other anomalies present. The man seems to convey how it feels via your phone, it's probably not purposely trying to communicate or whatever it's doing, but just a side effect? Such as the necrophagia part. I think it also connects you? Or maybe these parts are hallucinations induced by it's presence, what I'm talking about are the incessant, breathing, and the leaves falling parts, I think it's causing you to hallucinate and think about how it's incessant, hearing and feeling breathing, and seeing leaves.
Now onto your friend! I think your friend is also capable of infecting your phone, and is also an anomaly of sorts, since they're aware of things resetting, and I think them acting dumb about your window being broken is to keep the loops intact until they can find a solution. When they do find a solution, they leave themselves vulnerable....
The endings in my opinion show off more of what the man's goals are and how a time loop is in play and how your friend isn't human, but not stronger than the man.
Bad ending:
You suck at vocab, which doesn't really... matter? But your friend nor your mom is unable to help you leaving you to your fate (at least until the loop restarts).
There are two things I think about this ending:
Either your friend is kidnapped (which explains why they quit responding) and is murdered, which is why you smell mushrooms and something rott...en and the door is locked, shown by the audio of a what sounds like a doorknob being use but not opening, and then what we can only assume is the man killing you and eating your corpse (necrophagia is one of the words in the game for a reason. I think this is the most likely scenario.
or
The man is essentially a walking corpse that can mess with electronics, a very common spirit or whatever ability. It's stopping you from receiving messages or contacting anyone for that matter. See above to what I think it does.
Good ending:
Your friend showing up before the man gets to them (this ending is what makes me think about the former part of my theory on the bad ending is the most likely scenario) and is able to drive them away. The broken window's significance makes me think the man is aware of the time loop and cracks as if to say, "I will get you next time." and then scenario loops once more, which I think is your friend's influence whether they're aware of what they're doing or not?
Good ending (barely): I'm thinking with this one, any sort of outside influence can drive the man away, but only your friend is capable of fighting back against it sorta. Window theory stays the same.
Final ending:
This is where it culminates, your friend has finally figured out this thing isn't giving up, and decides to figure out how to stop it.
Due to being otherworldly, and able to look into your phone too, they can sense it's attraction to phone and is using it to track you. They do some digging, yada-yada, and discovers this.
They try telling you in the final texts, but is stopped by the man outside, and you seem to be left to your fate again and yet another time loop, but your friend instead decides to risk it and leave themselves open to leave a message via the app telling you to turn off your phone.
It works and the man is gone, bad news is, it has a new target, tracking your friend via their involvement in your phone and following the trail. It's hunger is now directed to them. I think the man outside will also employ a new tactic, one where turning off their phone won't be so effective.
And that's my theory! To anyone who reads, I hope you enjoyed my rambling, and feel to criticize it, or suggest something else. Some other stuff I loved about this game is that during the third set of all the difficulties, most if not all the words are related to your current situation, such as:
Nadir: lowest point, it's the lowest point for your safety
Macilent: emaciated, the man outside is emaciated and needs food, which happens to be you
Necrophagy: corpse eating, do I really need to explain this one?
There are others, but these stuck out to me the most.
Loved the game!