Apple - enabling virtual machine Guest Additions on a Mac host and Mac guest

I'll quote a previous answer of mine:

From Virtualbox Manual : Chapter 14:

14.2. Known Issues
Mac OS X guests:
VirtualBox does not provide Guest Additions for Mac OS X at this time

[snip]

From the content of this, I wouldn't hold out for such support in VirtualBox any time soon.

The situation really has not changed since then.

There is some discussion of a workaround involving the built in OS X sharing functionality and configuring your network setup to work, but nothing well defined.

Guest Additions are available for macOS starting with VirtualBox 6.0

Per the change log, macOS/OSX Guest Additions are at "initial support" meaning features/functionality could be limited.

Resolving this question involves having guest additions on a MacOS guest. If, instead, you want to install guest additions from a MacOS host onto a non-MacOS guest:

You can copy the guest additions iso from your machine.

First locate VirtualBox in the Application folder, right-click it and select show package contents. Then browse to Contents/MacOS/VBoxGuestAdditions.iso. Copy it to a location that accessible when browsing for files.

After that, you can insert the device by clicking Devices > Optical Drives > Choose disk image... and then choose your copied iso file.

Then you can run your iso from within your vm and install the guest additions.