I've encountered many people being oblivious to the way web application rendering works. Initial rendering is an important topic, especially in the context of SEO and performance, so brew yourself a nice cup of tea and pay attention.
Now, I will be talking about customer-facing web apps. When building internal tools or toy apps, it does not really matter. Just pick whichever is the simplest to implement (which is CSR in most cases).
There are three stages of the initial app render: