Is the battery upgradeable?

Yeah, that's the basic problem with Lithium Ion batteries; there aren't really any significant advances in energy density for them so the capacity is pretty much a direct result of their size. Meaning the only real way to get more capacity is to make the battery bigger. In phones, that space is extremely limited; batteries are made to take up as much space as there is so they'll last as long as possible and there's no extra room for expansion in any smartphone I've ever seen.

You may have seen batteries advertised as "gold high capacity" batteries, but those basically turned out to be a scam; they really didn't last any longer than a normal battery. That myth was busted by our own @oldturkey03 in his article on the myth of high capacity batteries.

The myth of the "high capacity" batteries - iFixit Repair Guide

As batteries age, they lose capacity so if your battery has been around for a while it's possible it's just not holding as much charge as it should. If that's the case then replacing it could take it back up to where it should be.

You don't say whether it's a Samsung Galaxy Note 8 or a Xiaomi Redmi Note 8, so I can't tell whether your battery is from 2017 or 2019, but if it's the original battery from either phone it's quite likely that it's time for a new one.