Better understanding installing plugins

I am trying to understand installing vue plugins better. Recently I had this issue when I was trying to install and use the Vue Dayjs plugin. The git readme had me install with npm then add:

import Dayjs from 'vue-dayjs';


When I look at tutorials online they use dayjs() but I get an error when I do that. Instead I discovered that I had to use this.$dayjs() in my component.

This might be a newbie question but could someone explain what is happening here and why I couldn’t use dayjs()?