Many operating systems (Windows, Linux, MacOS, FreeDOS, etc) can be downloaded as a ISO image.
Such ISO images were burnt to a DVD disc, but usually tools like Rufus are used to prepare a USB flash drive with one ISO.
If you want to install different operating systems, you would then have to use one USB drive for each operating system.
Ventoy allows you to store multiple ISO files of operating systems on a single USB drive and allows you to select which ISO you wish to boot.
You can customise and enhance Ventoy with various plugins and change the look and feel of Ventoy's main menu.