Session is all the changes you made after opening the site on all of the inputs & that changes will be saved in your local Computer if you click on the click to autosave this session
to access your session you've to go to settings>Load sessions>Load & then click on load associated with your desired session to open
Downloading is to save session in the form of file into your disk which will be visible as json data of whole session
The session file will be saved as chnp-session-json extension which can be opened in notepad
You can refer official repo to do changes into your file. you can also use OOP based realtime editor library in js from that repo