SchUseStrongCrypto disable SSL3 protocol and TLS weak ciphers (RC4/DES/MD5...), and enable/set set ServicePointManager.SecurityProtocol to all TLS protocols (including Tls 1.3 if supported)
SystemDefaultTlsVersions set ServicePointManager.SecurityProtocol to SystemDefault, which inherit the defaults from the Windows Schannel DisabledByDefault registry values
if DisabledByDefault is not defined for the protocol, then it depends on the OS version and installed updates for older versions
https://learn.microsoft.com/en-us/windows/win32/secauthn/protocols-in-tls-ssl--schannel-ssp-
with my test on Windows 8.1, neither of SchUseStrongCrypto and SystemDefaultTlsVersions registry values will disable older TLS versions, and it can still be negotiated and used
you can check in powershell for the current value
[Net.ServicePointManager]::SecurityProtocol
and test against specific TLS version checkers
Invoke-WebRequest https://tls-v1-0.badssl.com:1010
Invoke-WebRequest https://tls-v1-1.badssl.com:1011
Invoke-WebRequest https://tls-v1-2.badssl.com:1012
or
(Invoke-WebRequest https://www.ssllabs.com:10300).Content
(Invoke-WebRequest https://www.ssllabs.com:10301).Content
(Invoke-WebRequest https://www.ssllabs.com:10302).Content
(Invoke-WebRequest https://www.ssllabs.com:10303).Content