How to open Activity Monitor
You can open Activity Monitor from Applications > Utilities, or search for Activity Monitor with Spotlight. If you only need to force quit an app, the Mac shortcut is Option-Command-Esc.
Task Manager on Mac
What is Task Manager called on Mac?
Task Manager is called Activity Monitor on Mac. It is the built-in app for viewing running processes, CPU, memory, energy, disk, and network activity.
Short answer: open Activity Monitor when you are looking for Task Manager on Mac.
Apple's Mac User Guide maps Task Manager to Activity Monitor, and Apple's Activity Monitor guide describes it as the place to view process and resource information.
What Activity Monitor does
Activity Monitor shows apps and processes running on your Mac. It lets you inspect CPU, memory, energy, disk, and network activity, and it can help you quit a process that is not responding.
How Callsign helps with the name mismatch
Callsign creates a visible alias named Task Manager in a folder you choose. When you use that familiar name, it opens Activity Monitor. The alias is a normal visible file, not a background service, launcher replacement, or private file index.
Related Windows names
Performance Monitor and Resource Monitor also map naturally to Activity Monitor for Callsign's purposes, because they are about resource usage and running processes. For broader system details, Device Manager maps more closely to System Information.