I flew only in childhood about 20-25 times, from about 1980 to 1988. Mostly on the TU-154. In the vast majority of cases, passengers were boarding from the starboard side.
Indeed, although this is not true for all aircraft, most models common in civil aviation have the main passenger entrance on the left. The practical explanation is that luggage and food are loaded on the right side (luggage hatches are located on the right), so it would be irrational and unsafe to let passengers on the right side too.
There is also a version explaining this location by the successive connection between aviation and shipbuilding: the passengers entered the stairs to the port side (backboard) of the ship, and not to the starboard side (starboard).
Another version connects this with the fact that the commander of the ship always sat on the left, so the location of the door for entering and exiting passengers on the left gave him a better view and allowed him to steer the plane more accurately on the runway - initially the plane reached the airport terminal itself and unloaded passengers already there.
By the way, the entrance is usually located on the left and most often in the front of the plane - although there are doors not only there. An interesting study is connected with this circumstance, according to which, on flights with business class, passengers in economy class are almost 4 times more likely to have outbursts of rage. At the same time, on planes with such an arrangement of doors, the chances of such an outbreak among economy class passengers increase 2.18 times - after all, when boarding, they will have to pass by business class passengers who have already taken their seats.
Modern passenger aircraft have exits in both directions. First of all, so that there was a choice of a safer direction in case of an emergency exit from the aircraft. During regular embarkation / disembarkation, the left side of the aircraft is used because airport services work on the right side, in particular, they load luggage through the luggage hatches. Exit on the left, luggage hatches on the right - a solution for unification and uniformity, so that the same people can serve different types of aircraft using the same algorithm.
A long time ago, when airlines such as American Airways and United were just starting to buy passenger aircraft, the entrance could be either on the right or on the left. I have a friend, Ashley, who still flies a vintage plane, and there is an entrance on the right. But somewhere in the late 1940s, perhaps to streamline production, the aircraft companies decided to create a single standard and began to enter from the left on each aircraft. Why left and not right? I assume that the question was solved with the help of a game of poker, or even easier - tossing a coin.