How could Nokia come into existence and start making mobile phones even after it was merged into Microsoft?

Nokia was never merged into Microsoft, what merged into Microsoft was Nokia's Devices and services business.

Nokia left with 3 business by then. First was Nokia Networks, second was Mapping business (HERE maps) and third was Advance technologies (now known as Nokia Technologies).

Nokia signed a 'non-compete agreement' as part of Nokia-MS deal and according to this agreement Nokia could not make phones with 'NOKIA' branding until 31st Dec. 2015. (Niether can Nokia license its brand to any other company).

On 18th May 2016, two deals were announced, first between Nokia and HMD Global ( a newly start up formed by many old Nokia veterans, based in Finland), in this deal Nokia (technologies) licensed 'NOKIA' brand and many important patents to HMD for making Nokia branded devices for next 10 year. Second deal happended between MICROSOFT-HMD-FIH Mobile (a foxconn subsidiary), in this deal, Microsoft sold their feature phone business to HMD-FIH for $350 million. On 1st Dec 16, HMD started their operation and till now have launched several Devices like Nokia 150, 3310(2017), 3,5,6 and 8.

So, this is how Nokia came back in phone market.

*Nokia still exists as an independent company with nearly $30 Billion revenue, and falls in Fortune 500.

Who told you that? Nokia is a group holding with different businesses that operate under the Nokia brand. In 2014 Nokia sold its entire devices and services business that designed, developed and manufactured smart devices and mobile phone. So Nokia was neither sold nor merged. The new devices are designed, marketed and sold by HMD , a company formed by ex Nokia employee. This company has been given the license by Nokia Corporation to use Nokia brand name on their devices for next 10 years.

Hope i could answer your question. Today nokia businesses are:

  1. Networks
  2. Virtual Reality
  3. Digital Health
  4. Technology developement and licensing
  5. Brand licensing

It's about marketing strategy, do you think even after everything all the setup of Nokia (old Nokia) will be there. Well, it is all refreshed Nokia team, who are working now in the Nokia Eco-system.

We all know how popular Nokia was before the adoption of Android OS. The new Nokia team, just trying to take the brand name(Nokia) advantage. Rather than I don't see any Nokia like feeling in current smartphone. I mean nothing different to see in the new Nokia device, as the other brand also offering the same hardware and feature at cheaper price...

Does have the potential to attract more users than

As many said, Gmail was the most disruptive innovation Google made. Do you remember mails before Gmail? You had 2MB space, you could create folders to put your most important mail in and.. almost all. You have to empty your

Can I replace a non-removable battery in a mobile?

Yes. In most cases a non removable battery can be replaced. It will require taking the device apart and removing the old battery. I would look up a guide for your phone on a website like iFixit so you have a guide to work from. Some batteries will