A shared folder into a VM can be done two ways:
1. With Virtualbox Shared Folders. These are set up in the VM's Shared Folders settings, where you pick the host's folder to share. The VM requires Guest Additions installed to see this shared folder. See section 4.3 in the Virtualbox manual. (Also see "problem" below.)
2. Share the folder using the host OS's built-in network folder-sharing system, and see the folder in the VM over a Host-Only or Bridged network. Use the VM OS's built-in methods to see the host's shared folder over the network. No Guest Additions required.
Problem: Apple's EULA for Mac OS's in a VM over the years has somewhat restricted interest in developing Guest Additions for Mac VMs. So I'm not sure if the Mac Guest Additions has Virtualbox Shared Folders.
You can certainly try Virtualbox Shared Folders and Guest Additions. For a more sure-fire setup, use method 2: network the VM to the host using Host-Only, then use Mac OS's usual shared folder settings on both host and VM. If you want internet in the VM also, set a NAT adapter as the first adapter, and Host-Only as the second adapter. Bridged can also be used instead of the Host-Only/NAT combination, but Bridged doesn't always work when the host is using Wi-Fi for its network. See
Virtualbox Networks: In Pictures.