By default, attachments are stored outside the database, as binary files on the server.

The default location for databases hosted on a server are defined in the server configuration file.

From https://www.odoo.com/documentation/15.0/developer/misc/other/cmdline.html#cmdoption-odoo-bin-D

-D,--data-dir

directory path where to store Odoo data (eg. filestore, sessions). If not specified, Odoo will fallback to a predefined path. On Unix systems its one defined in $XDG_DATA_HOME environmental variable or ~/.local/share/Odoo or /var/lib/Odoo.

A database specific location can be set via the System Parameter ir_attachment.location as discussed at https://www.odoo.com/forum/help-1/where-are-document-attachments-stored-529 although the default location for attachments has been the database since version 8 (they were stored in the database prior to that).

If you do not provide a location in the configuration file or the database, Odoo will default to:

.local/share/Odoo/filestore

this folder is created in the home folder of the User who is starting the Odoo process.