Without an account, your files are kept in this browser, on this device, and nowhere else: 8 MB of room, up to 2,000 files and folders. Nothing is uploaded until you use a tool, and then only the file the tool is given, which the server does not keep.
A browser may clear what a site has stored. Safari does after about a week without a visit, and “clear browsing data” always does. Download what matters. The workspace asks the browser to keep its storage once you have saved something, which most browsers then do.
files too big to keep
A scanned book can be tens of megabytes, so a real scan often will not fit in 8 MB. Drop it in anyway: it opens without being kept, its tools work on it as it is, and only the small thing a tool makes is saved.
demos and automations
These two folders are the site's own and cannot be changed. The demos folder has MIDI files, sheet music and scanned pages to try the tools on, and the automations folder has the tools themselves, as the flows they are. A tool run on a demo puts its result at the top of your files, and “copy to my files” makes a copy you can change.
zip archives
Right-click anything to compress it into a .zip beside it. A .zip's tools offer to extract it. It unpacks into a folder of its own, named after the archive, or, when everything in it is in one folder already, as that folder, so songs.zip does not become songs/songs. A zip dropped on the preview can be extracted without the zip itself being kept.
An archive lands whole or not at all. How much it unpacks to is read from it first and checked against your room before anything is unpacked, and each file is then held to that size, so a small zip that claims little and unpacks to a lot is stopped, not stored. Up to 128 MB and 2,000 files and folders go in or come out at once. Files locked with a password, links, and anything whose path would climb out of its folder are left out, and the tasks page says which. What a Mac or Windows leaves in an archive (__MACOSX, .DS_Store) is left out without a word.
names
Names keep the capitals you give them, and Étude.mid and étude.mid are the same file. A name cannot hold / \ : * ? " < > |, begin or end with a space, or end with a full stop: the rules of the fussiest computer it might be downloaded to. Folders can go 8 deep.
with an account
With an account, your files are kept on the kobimusic server instead of in your browser. You get 512 MB of room, and the files are the same on any device you sign in on. When you make an account, the account page offers to move the files from this browser into it.
The files are encrypted on the server's disk. The server can still read them, since it needs to in order to run the tools on them. So this is not end-to-end encryption.
You can make an account using a password or Google. An account uses one or the other, not both. A link confirms the account's email, and until then it gets 5 credits a week instead of 30. If you forget your password, the sign-in page can email you a link to set a new one.
Every other cap is on the limits page. With a whole collection, for institutions says how to work in batches when it is more than the room.