A passenger of an international flight will have to go through customs control at the airport of departure. The main task pursued by the border control is to prevent the limited export / import of cash, valuables, goods of certain categories without inspection by customs officers.
In some countries, customs procedures for arriving passengers at many international airports and some road crossings are separated into red and green channels. Passengers with goods to declare (carrying items above the permitted customs limits and/or carrying prohibited items) go through the red channel. Green-edged label Passengers with nothing to declare (carrying goods within the customs limits only and not carrying prohibited items) go through the green channel.
These passengers are subject only to spot checks to save time. However, if a passenger going through the green channel is found to be carrying goods above the customs limits or prohibited items, he or she may be prosecuted for making a false declaration to customs, by virtue of having gone through the green channel. Australia, Canada, Ireland, New Zealand, the United Kingdom, and the United States do not officially operate a red and green channel system; however, some airports copy this layout. Airports in some EU countries like Finland, Ireland or the United Kingdom also have a blue channel. As the EU is a customs union, travellers between EU countries do not have to pay customs duties.
Value-added tax and excise duties may be applicable if the goods are subsequently sold, but these are collected when the goods are sold, not at the border. Passengers arriving from other EU countries go through the blue channel, where they may still be subject to checks for prohibited or restricted goods. In addition, limitations exist on various tobacco and alcohol products being imported from other EU member states, and if those limitations are being exceeded use of the blue channel would be inappropriate. Luggage tickets for checked luggage travelling within the EU are green-edged so they may be identified. UK policy is that entry into a particular channel constitutes a legal declaration. In most EU member states, travellers coming from other EU countries can simply use the green lane. Customs declaration Lodging a customs declaration is the act whereby a person indicates the wish to place goods under a given customs procedure. In general this task is performed by the owner of the goods or a person acting on his behalf (a representative). It may also be performed by the person having control over the goods. These persons may be individuals or companies, as well as in certain cases associations of persons. The declaration is to be lodged with the customs office where the goods were or will shortly be presented.
Source for information:
https://visasam.ru/samotur/rules/peresechenie-granicy-rossii-na-poezde.html
https://info.wikireading.ru/31391
https://vokzal.ru/blog/na-poezde-za-granicu-pravila-proezda