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The comments (thanks @Jan and @Kevin) under the question provide the correct answer:
Note that in just about every country on the planet, you are responsible for complying with customs and immigration laws, including obtaining a visa/entry clearance/whatever-they-call-it-there if applicable, regardless of whether or not there is a passport checkpoint.
The checkpoints are only at the airport for reasons of security and convenience. If the checkpoint is absent or unstaffed, you still have to find an immigration official and submit to whatever formalities the law requires.
The information about Belarus is on the pages of the Belarus embassy in Brazil, for example.
Since this information is not easily found through a Google search and, as a traveller, it is your responsibility to check these things before your departure, in order to reliably get the most up to date information you should directly contact a representative of the embassy for any country you plan on visiting.