IPv4 addresses run out few years ago. But everywhere I can see mine or someone elses IP address it's in IPv4, not IPv6. I understand that world switched to IPv6? So why we can see address in older format? Does that mean somewhere someone can have same address as I? How can that work?
ISPs asign one IPv4 address to a customer, and your "WiFi box" acts as a router/NAT box/ DHCP server doling out addresses from a private range internally. It is possible that the "outside" IPv4 address is itself in a private range, leaving your ISP through a mere handful of IPv4 addresses via NAT (real one, your box really does PAT, port forwarding through one IPv4 address).
Yes and no -- the same IPv4 adress adress can be used by other computers. But only "locally", not globally.
IP-adresses are often translated using things like NAT - network adress translation.
Every computer I have seen has an internal IP-adress of 127.0.0.1. This adress can be used inside the computer, not from the outside.
On my LAN my computer currently has the IP adress 10.0.1.30. I would expect millions of LAN connected computers to have the same adress.
Before going to the internet, the adress is translated by my access point into something else, assigned by the ISP. The access point keeps track of which of the several computers on the LAN that has started the connection and that has sent a request and will receive a response. In effect all computers on my LAN has the same external IPv4 adress. A further translation or "packaging" of adresses may be done by the ISP.
My adress on the "global" internet is currently IPv4 and is unique, no other computer has that adress. The adresses are in limited (exhausted really) supply. So we will be able to live only a while more on IPv4.
The idea of IPv6 is that every computing device will have its own, unique, adress. I guess this was the original idea between IPv4, but then the number of devices simply exploded.