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Hello,

first of all, when you're restoring a backup there's no need to delete all the existing folders, unless you're installing a different version of Joomla (ie Joomla 3 on top of Joomla 4 or viceversa) or you're downgrading the existing installation (ie installing Joomla 4.1 on a Joomla 4.3 site). In those cases you have to delete everything since Joomla will move and delete some files in newer versions, if you do not start from a blank state you will have issues.

That being said, if I got it correctly (your ticket is missing all the punctuation so it's a bit hard to understand), your problem is that you do not have enough space on your server. My suggestion is to take a backup of all your dev sites, store them outside your server (ie locally or upload them to a cloud storage engine), delete some dev sites to make space and then restore them.
I suspect you might have some cruft that has been accumulated over time, like old logs, old backups or full copies of your site

Hello,

even if using a different subdomain is always the best option, you can restore your new site inside a folder of an existing site. You can even use the same database of another test site, as long as the table prefix is different.

You will end up with something like this:

test.example.com
test.example.com/restored

Joomla is clever enough to avoid storing absolute paths, so every site is contained in itself

Hello,

it depends if your main site is backing up your dev sites as well. Take the production backup and restore it locally. If you see the folders where the dev sites are stored then, restoring that backup of the main site would break the dev sites. Same thing if you're using the same database for all sites and you're backing up all sites' databases.

I would also recommend to back up all dev sites and keep a copy of these backups locally, in case anything bad happens.

No, no other sites will be affected.