Source Available < Open Source < Free Software
These terms have specific definitions, where each greater term is more specific than the lesser*.
SSPL is in the “Source Available” tier.
The OSI defines the term “open source,” and the FSF defines the term “free software.” The number one term of open source, greater than the availability of the source code, is the freedom to redistribute.
https://en.wikipedia.org/wiki/Comparison_of_free_and_open-source_software_licenses
* Free Software isn’t exactly a subset of Open Source. There are a few licenses which are considered Free but not Open: the original BSD license, CC0, OpenSSL, WTFPL, XFree86 1.1, and Zope 1.0.
Absolutely. The source of Windows is widely made available to innumerable third parties, yet I’ve never seen anyone claim that it’s open source.
I didn’t think the Windows source is widely available, only the compiled form.
.Net core is open source though.
A lot of large companies have access to the Windows source tree. It’s quite common.