Where is control transferred after executing a system call?

After executing a system call, control returns to the user space of the program, to the point following the call to the system function.

That is, the process transitions from kernel mode, where the system call was executed, back to user mode, continuing the execution of its code.

For example, if in Go you call a function to read from a file, after the read system call completes, control returns to your program, and you can work with the obtained data.