@DennisSun The reason there's no footer.tpl in Lavender is because it uses the footer.tpl inside Vanilla. The only reason you'd create new files in new templates is if you wish to change the functionality of Vanilla. Hence Lavender.
It's a sort of hierarchy system, Magento uses it, Simple Machines uses it. You will have your "core" theme. Which is the default. In this case Vanilla, if no footer.tpl is defined in Lavender, it will be pulled from the core theme instead. If you wish to overwrite it, then just go into the Lavender templates folder, nano footer.tpl, copy the code from the vanilla footer.tpl into the lavender footer.tpl, and add your changes.
Making changes to core is never a good idea, as you've made potentially breaking changes to your one and only fallback.
Hope that more explains the process.