So, let's go over the architecture basics for the backend. First off, we're going to go with an industry standard way for modern backends and frontends to communicate: REST services communicating using JSON data. This has taken over the industry for many different reasons, among them being that it's fast, lightweight, goes over HTTP which is routable and non-routable right through firewalls solely via configuration, and is all easy to secure. IF you pay attention to detail and design and code and configure well, that is.

Remember, most banks and art museums don't get physically robbed. MOST. Unless you're sloppy and don't pay attention to detail*. And just like with robberies in real life, they usually start with known small security failures and get built on top of over time.

(*see the robbery of the Isabella Stuart Gardiner Museum here.)

The data, both of animals for adoption and of people submitting profiles to log in and be considered as clients to adopt pets, will be placed in a database.

All meat and potatoes stuff. But, just like with real world meals, it all depends on how you prep, season and cook the meat and potatoes that determines how good the meal is. The diff between tough mystery meat and instant potatoes and filet mignon and au gratin is smaller than you think. Keeping details in mind and where you want to wind up eventually makes all the difference even from your first development steps.