You can use mnemonics🧠 to memorize anything more quickly... For example,
Shift
means "backwards",
Ctrl
+
T
is "new browser tab", so
Ctrl
+
Shift
+
T
must be "old browser tab".

Mac

Effect

Shortcut

Demo
Copy
Paste
Cut (copy + delete)
Undo

Effect

Shortcut

Demo
Spotlight search - open any program or file
Open finder
Mission Control - view all open apps in a grid
Close system dialog boxes / pop-ups
Quick-switch programs

Hold
command 
, then tap
Tabtab symbol
, press the Tab key, then press
or
until you get to the app you want, then release
command 
.
Esc
to cancel.

Convenient for rapidly switching between two programs.

Effect

Shortcut

Demo
Hide an app
Quit an app
Close all open windows for an app (does not quit the app)
Force an app to quit
Task manager - force-quit programs (mini version of Activity Monitor)
Activity Monitor - diagnose problems like slow computer

Click the column headers to sort activities by usage of CPU, Memory etc. to find problematic activities.

To improve performance, check your Startup programs and disable any unnecessary activities.

task manager

Effect

Shortcut

Demo
Maximize window
Minimize window

You can set an option in Dock & Menu Bar preferences to have a window minimize when you double-click its title bar.

Snap window to sides

Try it with one window on each side for side-by-side mode!

Effect

Shortcut

Demo
Create a space

You can create up to 16 spaces
Tip: Make your spaces unique by assigning a different desktop picture to each one. Just assign System Preferences to all desktops
.
Delete a space

If the space contains open windows, they are moved to another space.
Move between spaces (virtual desktops)

On a trackpad, swipe left or right with three or four fingers. On a Magic Mouse, swipe with two fingers.
or
Enter Mission Control, move the pointer to the top edge of the screen, then click a space in the Spaces bar.
Move an app window between spaces
Task View: perform any of above using mouse
Show a window or program on all desktops

Effect

Shortcut

Demo
Copy screen clipping to clipboard
Screen recording or screenshot menu
New sticky note from selected text
Show the desktop
Show all windows of the front app
Lock the computer
🎈 Emoji menu 😎✨

HEY I'm a more info over here

Custom shortcuts

Effect

Shortcut

Demo
Something missing? I'm not an expert Mac user — recommendations are welcome!