Plesk360 and Imunify360 are both installed on a server and both use 360 in their name. That's where the similarities stop. But confusion is understandable. They are however very different products and serve a very different purpose. Both work well on Ubuntu.
Plesk 360 can ben considered a cloud monitoring tool for Plesk serves. It can keep track of resource usage like CPU and memory while also monitoring processes.
Imunify360 on the other hand is security tool. It helps to scan for viruses, malware, vulnerability exploitation and port scanning among many other things.