It's funny to me the number of developers who have effectively forgotten that Microsoft exists, and that it's possible to have your entire stack be provided by one company who directly sells it's software for profit.
Perhaps the parent's situation ("software that runs within very secure financial networks" cf https://news.ycombinator.com/item?id=24747781) prevents them to rely on an external service?
We cannot rely on anything outside the client's secure network. There are a few exceptional items that are allowed to talk to the internet, but our system's data persistence layer is not one of them.
Effectively, our client's operations cannot rely on cloud services and all of the related last mile connectivity into their infrastructure. If AWS/Azure/et.al. go down, many of our customers are still able to continue operating without difficulty.
C# 8.0 / .NET Core 3.x / AspNetCore / Blazor / SQLite
Of these, SQLite is arguably the most stellar example of what open source software can provide to the world.
Everything else in our stack consists of in-house primitives built upon these foundational components.