For adding an entry to the boot menu for an OS, please see here
When all OSs use the same /boot/efi partition, systemd-boot will only automatically pick up three of them: Pop (new+old kernel), Windows and one of grub based distros (as Linux boot manager).
This means you need to create small loader files in /boot/efi/loader/entries for all to display. The process is very simple, you only need a name and the *UUID of the root partition.
Here's an example of the entry I've made for Fedora, in the file Fedora.conf.
title Fedora 33
linux /EFI/fedora/grubx64.efi
options root=UUID=62a337f0-ae6b-4d17-83bb-8f1b86345e20 ro loglevel=0 splash
Once you have all your little OS_Name.conf files, you do a quick check to see what their ID's are with bootctl list
Here's mine:
Boot Loader Entries:
title: Fedora 33
id: Fedora.conf
source: /boot/efi/loader/entries/Fedora.conf
linux: /EFI/fedora/grubx64.efi
options: root=UUID=62a337f0-ae6b-4d17-83bb-8f1b86345e20 ro loglevel=0 splash
title: Fedora 33 Rawhide
id: FedoraR.conf
source: /boot/efi/loader/entries/FedoraR.conf
linux: /EFI/fedoraR/grubx64.efi
options: root=UUID=a9309eb6-e4c7-4ecd-a98a-1a0ce6326299 ro loglevel=0 splash
title: Manjaro 20.2
id: Manjaro.conf
source: /boot/efi/loader/entries/Manjaro.conf
linux: /EFI/Manjaro/grubx64.efi
options: root=UUID=e6d32909-cf87-45c2-bedd-f0a76ab9bbdb ro loglevel=0 splash
title: Pop!_OS (Pop_OS-current.conf) (default)
id: Pop_OS-current.conf
source: /boot/efi/loader/entries/Pop_OS-current.conf
linux: /EFI/Pop_OS-78c9787f-1d36-42e8-89bd-7b94b501afaf/vmlinuz.efi
initrd: /EFI/Pop_OS-78c9787f-1d36-42e8-89bd-7b94b501afaf/initrd.img
options: root=UUID=78c9787f-1d36-42e8-89bd-7b94b501afaf ro quiet loglevel=0 systemd.show_status=false rootflags=subvol=@ splash resume=UUID=78c9787f-1d36-42e8-89bd-7b94b501afaf resume_offset=7869696
title: Pop!_OS (Pop_OS-oldkern.conf)
id: Pop_OS-oldkern.conf
source: /boot/efi/loader/entries/Pop_OS-oldkern.conf
linux: /EFI/Pop_OS-78c9787f-1d36-42e8-89bd-7b94b501afaf/vmlinuz-previous.efi
initrd: /EFI/Pop_OS-78c9787f-1d36-42e8-89bd-7b94b501afaf/initrd.img-previous
options: root=UUID=78c9787f-1d36-42e8-89bd-7b94b501afaf ro quiet loglevel=0 systemd.show_status=false rootflags=subvol=@ splash resume=UUID=78c9787f-1d36-42e8-89bd-7b94b501afaf resume_offset=7869696
title: Pop!_OS recovery
id: Recovery-7827-FA9E.conf
source: /boot/efi/loader/entries/Recovery-7827-FA9E.conf
linux: /EFI/Recovery-7827-FA9E/vmlinuz.efi
initrd: /EFI/Recovery-7827-FA9E/initrd.gz
options: boot=casper hostname=recovery userfullname=Recovery username=recovery live-media-path=/casper-7827-FA9E live-media=/dev/disk/by-partuuid/a9fbe686-9f08-487c-9bc9-db094845b8c2 noprompt
title: Ubuntu 20.10
id: Ubuntu.conf
source: /boot/efi/loader/entries/Ubuntu.conf
linux: /EFI/ubuntu/grubx64.efi
options: root=UUID=6496be90-810c-4b5c-bc7f-624aa51c5d9d ro loglevel=0 splash
title: Windows Boot Manager
id: auto-windows
source: /sys/firmware/efi/efivars/LoaderEntries-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f
title: Reboot Into Firmware Interface
id: auto-reboot-to-firmware-setup
source: /sys/firmware/efi/efivars/LoaderEntries-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f
You can reboot to any of those by typing:
systemctl reboot --boot-loader-entry=Fedora.conf for Fedora,
systemctl reboot --boot-loader-entry=auto-windows for Windows
systemctl reboot --boot-loader-entry=Manjaro.conf for Manjaro
systemctl reboot --boot-loader-entry=auto-reboot-to-firmware-setup for the Bios.