IPv6 will eventually be required for IPv6 only services. China for example has a very limited quantity of ipv4 and a massive population. Also, IOT is going to drive IPv6 adoption, so if you want to be able to talk to IOT devices in the future you'll need functional ipv6.
If a service is on ipv4 there's no benefit to connecting to it using ipv6.
IPv6 has a very, very big address space. Every subnet in IPv6 (even point to point links) is 64 bits in size, which is 4 billion ipv4 internets worth of address space. i.e., your home network gets 4 billion times 4 billion ipv6 IPs. Wasteful? Sure, but it makes things like IPv6 auto negotiation, ipv6 address randomisation, etc, work properly and simplifies the global internet routing table significantly. And there are plenty enough ipv6 addresses to go around using the 128 bit address space. Like.... 2-4 per square cm of the earth's surface. NAT to try save IP addresses is simply not required.
Outside vs. inside networks are simple. And unique. Firewalling is no different to NAT (block all inbound, permit outbound, permit established return connections would be the sensible default), except you now have the option for full bi-directional communication if required without ugly state-tracking, in-flight packet mangling hacks - that actually reduce security to make VPNs even work.
Your inside network is just a subnet split off from your ISP's allocation. I.e., they will be unique, live, internet routable IPs. Common allocations are a /56 for a domestic connection, giving you up to 256 IPv6 subnets (each 4 billion internets worth of IPs) to use internally for different networks.
An enterprise will typically be given a /48 which is 65536 /64 networks for a business grade connection. Which is convenient - you could have 256 sites each with 256 different VLANs, for example - to better segregate traffic.
The ipv6 address space is large enough that if you were to generate a truly random 64 bit subnet has essentially no chance of collision with anything else. The address space is that large. But your internet routable allocation(s) will typically come from your ISP.
Without any configuration ipv6 defaults to link-local addresses (fe80:xxxxx) which will work with zero configuration between local machines by just plugging them in. The unique addresses are just determined by the network adapter's MAC address.
A bit of an illustration of size:
This article has an interactive graphic showing just how many IPv6 addresses are there. Grasp how huge the IPv6 address space really is!
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