Well, actually, in practice there are different things. And what is described in the first comment (gifts, travel payments, etc.), sometimes money is paid.
In general, according to the law, a doctor does not have the right to directly prescribe a medicine of a certain brand, he is obliged to indicate the active substance that must be used. For example, my doctor prescribed magnesium B6 vitamins. And I myself come to the pharmacy, choose which vitamins from which trading company I need to buy.
They don’t pay directly, but donate pens, robes with company symbols, small equipment. They take the necessary doctors to symposia, conferences and exhibitions at their own expense. Invitations to business lunches are possible, for consultation on issues of interest, at the expense of the interested company, of course. So, formally, money does not pass from hand to hand, but it does have a definite and rather strong influence.
As far as I know, this is prohibited by law. Company representatives can tell doctors about drugs, provide information, recommend, but not enter into a beneficial agreement with the doctor. And in the turbulent 90s, it seems, there really was such a practice.