The engine under a custom application. Invisible until something goes wrong.
A badly designed structure comes back to bite you with every extension. We deliberately spend time at the start on which data belongs where and how it fits together, because changing it later costs ten times as much.
Access control, encryption of sensitive data and logging belong in the design from the first line of code. Adding them afterwards usually means starting again.
Optimising for millions of users you do not have costs money and delivers nothing. We build for the current volume with room to grow, and scale up only when it is needed.
A question or project on this topic?