Need help understanding testing

I don’t have pretty much any experience in what should be tested in a code. I have an idea that every function should test just it’s own functionality and avoid testing the functionality of the function it calls. Is this a good approach? Here is an example what I mean by that. I have these functions: /// Used for adding endpoints to the pool, /// Complies with address family selected at start. pub fn add( self: *Self, io: std.Io, gpa: std.mem.Allocator, addr: std.Io.net.IpAddre...