How to install Guest addition in Mac OS as guest and Windows machine as host

I am trying to copy contents from my host machine to a guest machine, but for copying I need to install the Guest additions disk.

Or there is there another way to access host machine content on the guest machine or vice-versa?

Guest Additions are available for MacOS starting with VirtualBox 6.0.

Installing:

1. Boot & login into your guest macOS.
2. In VirtualBox UI, use menu Devices | Insert Guest Additions CD image...
3. CD will appear on your macOS desktop, open it.
4. Run VBoxDarwinAdditions.pkg.
5. Go through installer, it's mostly about clicking Next.
6. At some step, macOS will be asking about permissions for Oracle. Click the button to go to System Preferences and allow it.
7. If you forgot/misclicked in step 6, go to macOS System Preferences | Security & Privacy | General. In the bottom, there will be a question to allow permissions for Oracle. Allow it.

Troubleshooting

1. macOS 10.15 introduced new code signing requirements; Guest additions installation will fail. However, if you reboot and apply step 7 from list above, shared clipboard will still work.
2. VirtualBox < 6.0.12 has a bug where Guest Additions service doesn't start. Use newer VirtualBox.

1. In the guest Mac, open the Terminal and go for a reboot on the Recovery partition

sudo nvram "recovery-boot-mode=unused"
sudo reboot

2. Now you're in Recovery mode, enter the Terminal and do:

csrutil disable
spctl kext-consent add VB5E2TV963
nvram -d recovery-boot-mode
reboot

3. Back in "normal" mode, open the Terminal, and do:

sudo mount -uw /
sudo chown :admin /System/Library/Extensions/
sudo chmod 775 /System/Library/Extensions/

4. Run the Guest Additions installer and go through the end (in principle, it goes through successfully)

5. Now in the terminal, do:

sudo chown :wheel /System/Library/Extensions/
sudo chmod 755 /System/Library/Extensions/
sudo nvram "recovery-boot-mode=unused"
sudo reboot

6. Again in Recovery mode, go into the Terminal and do:

csrutil enable
nvram -d recovery-boot-mode
reboot

You should be set.

Guest additions are not available for Mac OS X. You can get features like clipboard sync and shared folders by using VNC and SMB. Here's my answer on a similar question.

You can use SSH and SFTP as suggested here.

1. In the Guest OS (Mac OS X), open System Preferences > Sharing, then activate Remote Login; note the ip address specified in the Remote Login instructions, e.g. ssh [email protected]
2. In VirtualBox, open Devices > Network > Network Settings > Advanced > Port Forwarding and specify Host IP = 127.0.0.1, Host Port 2222, Guest IP 10.0.2.15, Guest Port 22
3. On the Host OS, run the following command sftp -P 2222 [email protected]; if you prefer a graphical interface, you can use FileZilla

Replace user and 10.0.2.15 with the appropriate values relevant to your configuration.