Yes, it is possible to put the installers onto USB. The size required is between 4 gigabytes and 40 gigabytes, depending upon which toolkits you put in. More space would be required if you put in the installers for Windows, Mac, and Linux (they share some files, so it would not be triple the space.)
The installer can be run directly from USB, but typically performance would be better by copying the files to hard drive (or better yet, SSD) first.
With the R2020a installer, the important part is that after you have authenaticated your license in the installer, then you will be able to select Advanced Options in the upper right side of the installer screen. Choose "Download only". On the screen that comes next, you can choose which operating system to bring in the installers for: you can select more than one operating system.
When you choose Download Only, the file you indicate as the location to download into, will have a setup.exe (for PC), an install app for MacOS, and an install.sh for Linux (if you choose those options.) The user would use the one appropriate to their operating system. Note that this is not the same as the installers that you would download if you were doing an installation over the internet: the one put into the download directory is able to install from downloaded files without necessarily needing to contact Mathworks (in the case that the user has a File Installation Key.)
