Downloading the Stage Tarball:

mkdir /var/lib/machines/gentoo-stage3-amd64 && cd /var/lib/machines/gentoo-stage3-amd64
wget https://gentoo.osuosl.org/releases/amd64/autobuilds/current-stage3-amd64-systemd/stage3-amd64-systemd-*.tar.xz
tar xpvf stage3-*.tar.xz --xattrs-include='*.*' --numeric-owner

Config file: gentoo-stage3-amd64.nspawn

[Exec]
Boot=1
PrivateUsers=off
LinkJournal=no

[Files]
BindReadOnly=/var/db/repos
BindReadOnly=/usr/src

Bind=/var/cache/distfiles
Bind=/var/cache/binpkgs
Bind=/var/cache/binhost
Bind=/var/tmp/portage

Set password:

systemd-nspawn --settings=no -D /var/lib/machines/gentoo-stage3-amd64
passwd
exit

make.conf

ACCEPT_LICENSE="* -@EULA"

FEATURES="getbinpkg binpkg-request-signature"

MAKEOPTS="-j20 -l20"
EMERGE_DEFAULT_OPTS="--ask --jobs=10 --load-average=20 --verbose"

journald.conf

[Journal]
Storage=volatile

Boot and update:

systemd-nspawn --machine=gentoo-stage3-amd64
emerge --metadata
emerge --deep --newuse --update @system
emerge app-shells/bash-completion x11-terms/ghostty-terminfo
systemctl poweroff

chroot

mount --types proc /proc /mnt/gentoo/proc
mount --rbind /sys /mnt/gentoo/sys
mount --make-rslave /mnt/gentoo/sys
mount --rbind /dev /mnt/gentoo/dev
mount --make-rslave /mnt/gentoo/dev
mount --bind /run /mnt/gentoo/run
mount --make-slave /mnt/gentoo/run
cp --dereference /etc/resolv.conf /mnt/gentoo/etc/
chroot /mnt/gentoo /bin/bash
source /etc/profile
export PS1="(chroot) ${PS1}"

to install/update grub: mount /boot in chroot!

flushing the environment during chroot

env -i HOME=$HOME TERM=$TERM chroot /mnt/gentoo /bin/bash