You can download VirtualBox binaries and source code from this website. Make sure to read and agree to the terms and conditions of the license before downloading. VirtualBox 6.1 is the latest version and will be supported until December 2023. The binaries are released under the GPL version 3, and the Extension Pack binaries are under the VirtualBox Personal Use and Evaluation License. It is recommended to upgrade the guest additions after upgrading VirtualBox. The User Manual is included in the VirtualBox packages, but you can also access it separately. There are older builds available as well. The VirtualBox sources are free under the GNU General Public License, Version 3. You can also check out pre-built VirtualBox VMs for developers on the Oracle Tech Network site.
https://web.archive.org/web/20230723100307/https://www.virtualbox.org/wiki/Downloads