Use Apple Diagnostics to test your Mac
If you think your Mac might have a hardware issue, you can
use Apple Diagnostics to help determine which hardware
component might be at fault. Apple Diagnostics also suggests
solutions and helps you contact Apple Support for
EtreCheckPro (free) - for power users
CleanMyMac X (not free, but excellent)
CleanMyMac X is all-in-one package to awesomize your Mac. It
cleans megatons of junk and makes your computer run faster.
Just like it did on day one.
Proven Malwarebytes technology crushes the growing threat
of Mac malware, including thorough malware, spyware, and
virus removal. Finally, cybersecurity smart enough for
Share your screen using Messages on Mac
Share the screen of another Mac
One important capability available to Mac owners is the
ability to screen share between any two Mac computers over
the internet. With the permission of both parties the
initiator can share remote owner's screen, sound and cursor
Siri "Launch Screen Sharing"
DFU (User Beware)
Everything You Need to Know - DFU Mode for iOS/iPadOS Devices
The DFU Mode in iPhones and iPads is an emergency tool
that's not designed for most people to ever see. It's hard
to activate, and the screen stays blank when you finally
manage to trigger it. And yet this obscure tool is essential
for resurrecting a dead iPhone, or rolling back an unwanted
DFU Mode is more powerful than Recovery Mode, and is
unsupported and undocumented for us, the end user. It lets
an iOS device connect and talk to iTunes (or its Finder
equivalent in macOS Catalina), without loading the
bootloader, or iOS itself. As its full name - Device Firmware
Update Mode - suggests, the mode is used to update the actual
firmware of the device, mostly to install a boot loader.
DFU Mode is hard to enter, requiring convoluted sequences of
button presses to enter. Even when you reach DFU Mode, the
screen remains blank.
Unlike recovery mode, which can be updated with a software
update, DFU is burned into the hardware, and cannot be
altered. But it's a powerful tool. Because it works at a
lower level than Recovery Mode, DFU lets you do things not
allowed in Recovery Mode.