A big part of it is built in batteries that are difficult to replace. My phone has a removable battery and has is on its third one now… still works fine and does everything I want it to, after 10 years of use.
It’s honorable that you struggle through 2013 Android, but using an internet enabled device that hasn’t received security updates in at least 7 years is a horrible idea.
Upgrade to a Fairphone at least, so you can keep replacing parts while also maintaining a base level of security.