The images do not have a default password as that would be a big security issue.
Instead, by using incus exec, you can get a shell into the instance without a password.
You need to have access to the incus tool to be able to get such a shell. That means that you either need to be root on the host, or as a non-root user, be a member of the incus-admin group.
There’s an alias, incus shell, which can be used to get a shell into an instance.
The following two commands are equivalent.
incus exec myinstance -- su -l
incus shell myinstance
If your instance has a non-root account (such as images:debian/12/cloud, username: debian), then you can get a non-root shell account with the following command.
incus exec myinstance -- su --login debian