Hello, I really need some help with my project : when I bring modifications to my lists (or if the player does), the lists just forget them when we quit the project or refresh it. Does someone know how we can save them on the cloud ?

Sorry no. According to various sources, cloud lists are not possible in Scratch. You can try creating many cloud variables to emulate a cloud list, tho.

You can make a cloud list but you have to to code it yourself!
You convert all the list items to numerical codes and put a ‘00’ between each one.
Then put this number into a cloud variable (wrap around multiple cloud variables if you need more than 256 numbers for the encoded list)
To decode, simply split the code stored in cloud into chunks (delimited by the ‘00’ character) and decode them individually, placing each in a new list item.
The new list will be a ‘cloud list’

Thanks for the help ! It is probably going to be long but I am motivated (I do not only want the player's username to be saved on the cloud, I want the player's improvement in the game to be saved.)

Note that cloud variable can only save 256 numeric digits each and you can only have 10 variables per project. That can make it difficult or even impossible to store a decent amount of data for many players. However, if you don’t have too many people using the project and you erase older saves when more space is needed, it should be possible to make it save for a reasonable amount of time.

Thanks a lot, yes, it does make it much more difficult.

Hey again everyone, I am very proud because I did manage to code a username saving on cloud system, though… Well the cloud variable used doesn't save the modifications she has been brought, sometimes she does but sometimes she just forgets that another username has been added.

this could help clear things up
https://www.youtube.com/watch?v=YGiiqX8Q5Fg