I have a question about the trasit to any country. If suppose I go to a country where London would be my transit, can my friends/relative come and see me in the Airport? Is that possible? If not what would be the steps to visit them?
As of late there is no way for anyone to enter airside area in the airport in most of the countries, so in order for you to be able to see them you will need to get landside or they need to be able to get airside, so in most connecting airports in Europe you will need to get a visa because you're exiting airside area or your friends will have to get tickets to get into it.
In places like London where you might fly into one airport and leave from another landside transit is allowed under a different visa requirements if staying under 24 hours (Landside transit visa in London Heathrow airport) if you can reasonable prove that you're staying in the country < 24hours you should be allowed to exit and see your friends.