How to check max allowed open files (ulimit) of a running process (or running app)?
You cannot do this in a way that is similar to Linux.
procfs, although common on many Unix-like operating systems, is not available on all Unix-like operating systems. It is not available on macOS.
The POSIX standard prescribes a getrlimit() system call for retrieving these limits for the current process. It is available on macOS as it is POSIX-compliant.
Linux on the other hand has implemented a system call known as prlimit(), which allows getting these limits for other processes than the current process. The system call is specific to Linux and not available on other Unix-like operating systems.
All in all this means that there's no direct API for retrieving that information on macOS.
However, you can retrieve the information by the way of inspection inside the other process. For example by attaching similar to how a debugger would attach (ptrace()), or by the use of dtrace. It is though possible for the other process to actively resist this measure.
Depending on your specific use case, which you have not revealed, YMMV.