Help: Should I separate the frontend and backend (middle-tier)? If so, how?

Question

Hi! I'm pretty new to add-in development and I'm not sure if this is the right place to post this.

Anyway, after (semi-successfully) building a prototype add-in for Outlook without any authentication, I decided it would be best to implement it and use the most modern method available: SSO.

Spent some time learning how it works and going through the tutorials and docs on Microsoft Learn.

However, I note that the template projects combine the client-side and server-side (middle-tier) code.

In my previous build, I had separated the frontend and backend into different, compartmentalized components. My assumption was that this would be the best way to keep things clean and easy to work with.

Now, I'm unsure how add-ins are supposed to work. Is monolithic architecture the norm or should I still be looking to separate my client-side and server-side code?

My initial leaning is still toward the latter. But I have had some trouble figuring out which components of the template are associated with the client-side and which are needed for the "middle-tier" to work.

Any advice or help for this would be appreciated!