Multisite - single files sharing info required please

Hi there,
I am building a mutisite environment (dont know if it is the proper word for it) where everyone
will share the same file system but will have their own database to build what they need to see.

I am wondering and trying to figure out the best way possible to manage users media, robots.txt,
and sitemap.xml.

Must it all be in a folder with a unique ID?? but then what about when there is loads of users, does this
still work?

Hope i explained my problem well.

Thanks for any help,

Regards,

Jerome