When you create a new project without any framework, the default web part content in the code will be added through innerHTML.
Technically there is nothing wrong with this approach, but you might run into a problem when you add more content to the web part afterward using the same method. In this case, the previously added content will be overwritten.
To avoid this behavior, you can use jQuery and the ‘append‘ or ‘prepend‘ methods to add your content. It works well but requires an external library.
Instead of using jQuery, there is a native method in the document object model of HTML available.