In the sense that most modern programming languages and compilers are open-source, sure, nothing outside the embedded world can truly be built without relying on open source.
There are still native shops that rely on very little open source, though at this point probably only in niches like gamedev or defence.
This is not true even in those spaces anymore. Games these days require libraries like SDL, or (increasingly) use engines like Godot.
Defense is a weird place, but open source is used quite a lot there, it's often required to do so and to record the open source consumed to produce a product. And often times, it must be commercial open source where you can get engineering support for the lifetime of the product's existence.
There are still native shops that rely on very little open source, though at this point probably only in niches like gamedev or defence.