SQlite as a database for web services had a little bit of a boom due to:

1. People gaining newfound appreciation of having the database on the same machine as the web server itself. The latency gains can be substantial and obviously there are some small cost savings too as you don't need a separate database server anymore. This does obviously limit you to a single web server, but single machines can have tons of cores and serve tens of thousands of requests per second, so that is not as limiting as you'd think.

2. Tools like litestream will continuously back up all writes to object storage, so that one web server having a hardware failure is not a problem as long as your SLA allow downtimes of a few minutes every few years. (and let's be real, most small companies for which this would be a good architecture don't have any SLA at all)

3. SQLite has concurrent writes now, so it's gotten much more performant in situations with multiple users at the same time.

So for specific use cases it can be a nice setup because you don't feel the downsides (yet) but you do get better latency and simpler architecture. That said, there's a reason the standard became the standard, so unless you have a very specific reason to choose this I'd recommend the "normal" multitier architectures in like 99% of cases.

SQLite is likely the most widely used production database due to its widespread usage in desktop and mobile software, and SQLite databases being a Library of Congress "sustainable format".

"Production" can mean many different things to different people. It's very widely used as a backend strutured file format in Android and iOS/macOS (e.g. for appls like Notes, Photos). Is that "production"? It's not widely used and largely inappropriate for applications with many concurrent writes.

Sqlite docs has a good overview of appropriate and inappropriate uses: https://sqlite.org/whentouse.html It's best to start with Section 2 "Situations Where A Client/Server RDBMS May Work Better"

The reason you heard that was probably because they were talking about a more specific circumstance. For example SQLite is often used as a database during development in Django projects but not usually in production (there are exceptions of course!). So you may have read when setting up Django, or a similar thing, that the SQLite option wasn't meant for production because usually you'd use a database like Postgres for that. Absolutely doesn't mean that SQLite isn't used in production, it's just used for different things.

Only for large scale multiple user applications. It’s more than reasonable as a data store in local applications or at smaller scales where having the application and data layer on the same machine are acceptable.

If you’re at a point where the application needs to talk over a network to your database then that’s a reasonable heuristic that you should use a different DB. I personally wouldn’t trust my data to NFS.