ISPs wouldn't assign addresses to the IoT devices with IPv6 either...

You can do it (by that I mean NOW) with DHCP-PD (prefix delegation). Router asks for a range of addresses from the ISP which it then assigns to the IoT devices on the inside.

But who does? Or what factor decides what my iot's device ipv6 will be?

Devices tend to self-assign IPv6 addresses using SLAAC, but the router would still have MAC-to-IP mappings in its neighbor table. Or some cases the router assigns it using DHCPv6.

Just to clear things up for people unfamiliar with IPv6, the ISP does have some input in this process: they give your router a unique prefix (basically a subnet) which the router delegates to the devices in your network.