first part sounds like https://www.mangago.me/read-manga/to_you_the_immortal/
omg yes, they really do share a similar concept and feel almost alike in some ways. but sadly, it’s not to you, the immortal. the one im trying to find is actually pretty old manga and it was just a one-shot. i remember it being short but really impactful.
but still thank you so much for helping and taking the time to guess! :)
oh i didnt see u were referring to a oneshot mbb. perhaps it's this one: https://www.mangago.me/read-manga/omoide_emanon/
it tells the story of an ancient entity that has existed since the very beginning of the earth. every time it dies, it is reborn into a new body, taking on a different form and living through another lifetime in a different vessel.
one day, a man goes on a trip and meets this entity, who is now in the form of a young girl. it happens either on a ship or on a train (i can’t quite remember which one, since i've forgotten that detail). they began talking and have a long conversation about many different things.
i randomly remembered this manga out of nowhere and it's been stuck in my head now im dying to reread it, id seriously appreciate it if someone could help me find the title again