Which apt package do I install to have virtual console that you access with the Alt-Fn keys?
No additional package is required, but systemd-logind is required to automatically spawn login TTYs when switching console:
systemctl unmask systemd-logind
systemctl start systemd-logind
To enable full logind features: G_AGI libpam-systemd
Not required for TTY spawn but for other packages probably expected when having logind available.