Create ZIP

Yes. The archive is built with fflate zipSync, which deflates the entries as it packs them. There is no compression level setting, so you get the default for every file. Already compressed input such as JPEG, MP4 or another zip will barely shrink, which is normal for any zip tool.

Not from the page. Each entry is stored under the file name exactly as it arrived, so the archive is flat. If you need a directory structure, name the files accordingly before you drop them, or build the archive with a tool that can read a folder tree.

The run is refused with "two input files are both named" and the name, and you are asked to rename one. Silently overwriting would give you a smaller archive than the files you selected, with no error and nothing to notice, which is worse than stopping.

archive.zip by default, and you can change it with the outputName option. The name is used exactly as you type it, so include the .zip yourself if you want the extension. Every entry inside keeps the file name it arrived with, so the archive name is the only one you choose here.

No. Every file is read into memory in your browser and the archive is assembled there. Nothing reaches a server and there are no accounts. Since the inputs and the finished archive are all held in memory at once, the total size you can pack is bounded by your own device.

No. There is no encryption option, so the archive is a plain zip that anyone can open. To protect the contents, encrypt the files before zipping them or use a desktop archiver that supports encrypted entries. None of the archive tools here encrypt anything, so anything sensitive should already be encrypted before it goes in.