Which is, coincidentally, exactly how it works if your LAN is made up of devices with publicly-routable IPv4 addresses as well, which happens in business/academic/military networks all the time.
They just want to watch some reels.
No they don't.
Most ISP boxes only implement the bare minimum of functions to make sure that youtube is available to the users. Which includes NAT, because otherwise youtube does not work, and does not include anything else.
Anyway, NAT is costlier than a firewall. It uses more memory, it requires rewriting packets on-the-fly, and typically if you're using embedded Linux (I'll assume that the vast majority of consumer devices for this are) then you're already using `iptables` or `nftables` to get NAT functionality. It is comparatively to set default inbound/forward drop policies.
But yes, I should have said "in my experience," since it's true that I only know the networking equipment of a few people in a small country with limited IPv6 rollout (my ISP does not provide it).
This is dependent on the firewall features of the NAT router, but at least its something, and the router provides one centralized point of protection from interent traffic.
In IPv6 typically every machine on a LAN is directly connected to the internet with a public IP address. Every one of those machines now needs a full strength internet firewall.
This complicates some intra-LAN communications, and removes the feature of having the LAN be a walled garden, mostly isolated from the interent.