When an user switches on Internet on his cell phone, does he get a public IP from the ISP or a private IP till the ISP and then ISP performs NAT to route data to the Internet?

When an user turn on Internet, a PDP session is created and an IP is assigned with the help of GGSN, which maintains session logs and does the charging with the help of PCRF. Mostly, Mobile users will get private IPs. Public IPs are rarely assigned to mobile customers due to shortage of available pools. Whether users shall receive private or public IP, all that configuration is done on GGSN. From GGSN, traffic is pushed towards ISPs. On the edge router, which is termed as Internet Gateway, it performed NAT44 or NAT46 (Depends) and then routed towards ISP network. 

In short, Private IPs are assigned to customers with the help of GGSN and NAT is done edge routers. Read about CGNAT, these days every ISP is implementing it to NAT millions of IPs all together. Happy Learning!

It depends upon the service provider and yes it can be a public ip or a private one but its not that simple lan network scenario.

there can be multiple nats before leaving that isp to lan

for eq. i get an ip 19.19.19.19 which is a public ip but its not going to reach outside internet as it is

may be isp will convert it into 45.12.1.1 then convertn 45.12.1.1 into something else . This is done due to the vast number of devices and devices changing there towers while moving from one place to another , that why we cannot have a fixed lan or only use private ip address range in lan .

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