Don't follow youtubers, their advice is redundant to the manual or this forum at best and ill-advised click-bait at worse.
EMMC will suffer wearout from Proxmox write load ( logs, monitoring data, internal config database). For this reason the installer don't support this:
Hi,
Proxmox VE is not made for install on USB/eMMC/CFCard or equal.
Proxmox VE have to be installed on a SSD/HDD. The OS will write some GB logs per day.
A USB/eMMC/CFCard or equal will not last long with this many writing.
You could first install Debian and then Proxmox packages ( see:
https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_install_proxmox_ve_on_debian )
but the emmc will have the same issues.
So you will need to use at least one ( better two if you want a mirror so system continues to work after a disk failure ) of the drive slots for the system.
Another thing to think about: RAIDZ is ok for bulk data but not a good fit for storing vm images ( see:
https://forum.proxmox.com/threads/fabu-can-i-use-zfs-raidz-for-my-vms.159923/ ) so you might need to reconsider your storage layout.
Despite this: ProxmoxVE will run on this device but the CPU and limited non-upgradable RAM will limit the amount of workloads. If you just want a nas + plex it might make more sense to install unraid or TrueNAS baremetal, both support installing to emmc, won't wearout it and can run Plex as docker "app". Another option would be OpenMediaVault. It's free software and will also run Plex but will also write to the emmc ( more than TryeNAS/unraid but less than ProxmoxVE ). All of these NAS systems also support running vms so if your planned workload is limited ( let's say file sharing, plex, maybe a HomeAssistant VM and some other containers ( paperless, arr-stack) that might be good enough.
Another option would be to install your favorite Linux ( e.g. Debian or Ubuntu ) and afterwards Incus or virt-manager for managing VMs and portainer or dock.ge for managing containers.
In general: These devices are great NAS boxes but I wouldn't use them as virtualization hosts.