Program icons can be prepared from pretty much any graphics sources, including SVG vector graphics, which is what svg to ico conversion represent. This is actually preferable, as it gives the option to create icons in several resolution as single file. Many graphics editors nowadays give users the tools to make icons, but in the past dedicated icon making tools.
You can even find web pages that offer web tools with simple graphics editors and icon making tools, so it's really not difficult.
Updated: September 27, 2023