How .NET reads a format string
A format string of one character is a standard format: d is the culture's short date pattern and N is a number with group separators. Anything longer is a custom format, read one specifier at a time. To use a single custom specifier on its own, prefix it with %: %d is the day of the month, while d alone is the whole short date.
Every result on this page was checked against .NET 8 itself, on tens of thousands of values, formats and cultures. The culture data (month names, separators, currency patterns) was exported from .NET, not typed in.