Hi Federica,
Where the recordings are stored depends of your BigBlueButton configuration, but certainly they are out of your Moodle server.
If you have your own server so the archives are in your BigBlueButton server. There is some documentation in bigbluebutton.org that you system administrator will find very useful. For the maintenance you definitively need them involved.
If you are making use of a paid service of BigBlueButton you need to talk to your provider. Depending how the BBB server was installed/configured your recordings can be into the BBB server or in a separated repository somewhere else in the cloud. Sometimes this raises concerns due privacy protection.
Now, if have just started testing BBB and you are still making use of the server that comes pre-configured by default then your recordings are stored in that server, but they will only last for 14 days. After that they will be automatically deleted.
Regards,
Jesus Federico