If you execute a binary without specifying the path to it, it will be searched from the $PATH environment variable, which is a list of places to look for the binary. From left to right, the first found one is returned.

You can use which cat to see what it resolves to and whereis cat to get all possible results.

If you intentionally wants to use a different binary with the same name, you can either directly use its path, or prepend its path to $PATH.