Apple used tiny batteries up until the last few years. Small battery = bad battery life = multiple cycles per day. Battery life has always been the Achilles heel of the iPhone. That all changed with the XR and the 11 pros this year.
Lithium ion batteries are end of life at 80%, so your mom should replace hers asap before it swells up!
In my experience it's Android that has the achilles heel. The many Android phones I had would get really warm and be dead by lunch at the worst, or at least around 15% by 8 hours. Standby. My iPhones over the years always got a day to two--three at best. Much better than any Android phone. Only time I managed multiple day life on an Android phone was when I used a Galaxy Note 2, but I had no Google account, disabled Play Services, and removed all the Google apps. Basically it did nothing in the background.
As for my mom's iPhone, I subscribe to the saying "if it ain't broke don't fix it" so there's no reason to replace it until it no longer carries the sufficient charge or swells up. By then I bet she'll be ready for an upgrade anyway. She's had the thing since 2015 coming from a Galaxy S3 that was horrid on battery in comparison. My father was the record holder for iPhone longevity however. He was still using the iPhone 2G up until his death in 2016. Only used it for calls and Safari. Screen was super dim at max brightness but held a charge enough to make use of it.