Open streetmap is a database that you can use to produce maps from. Open streetmaps the organization does not produce maps as a service directly. They do provide a map editor, which of course also renders the tiles and they host the central database that people contribute to with that editor. But their tiles are not distributed via APIs or dumps otherwise. The only thing that they provide is a database dump in various formats.

Mostly people either generate their own tiles from this (using a variety of OSS and proprietary tools available for this) or use companies such as mapbox, maptiler, etc. that do this for them. Typically the commercial options bill using a metered per request model. This can make using maps quite expensive. Ever since Google raised their prices substantially a few years ago, things like Mapbox have become a lot more popular. Mapbox and similar products are also not cheap.

What pmtiles does is dramatically lowering the price and complexity for self hosting your own maps to basically the raw CDN network cost plus a small amount of overhead for storage and computation. The tiles are generated using e.g. lambda functions from a single pmtiles file on demand and then cached in the CDN. Any subsequent loads of the same tile are cache hits. So, especially for busy websites and apps, the savings can be substantial.