Humanoids start out with disadvantages in non-humanoid societies. All but the most enlightened civilizations consider humanoids to be monsters. Centuries of competition, violence and warfare has made humans and humanoids natural enemies, striving for the same resources. Truth became legends, and legends bred fears that haunt both sides, filling their heads with truths, half-truths, and lies. But humans are more numerous, more advanced, more established. They are winning the battle of dominion over the world. For better or for worse, though there are still vast stretches of untamed wilderness, it has become a human world.
For this reason, humanoids find themselves at a disadvantage. When they leave their tribes to find their own path in the world, it inevitably crosses into human civilization. Humanoids are strangers to human civilization (or even demihuman, for that matter). They know it only as something out of tribal legends, or from the scary stories told around the evening fire, or from the skirmishes their tribe may have had with a town or village in the past. They do not know the customs. They do not know the social etiquette. They probably do not understand many of the "advanced" conveniences that dominate civilized life.
It is up to players and DMs to work together to stress a humanoid's unfamiliarity with civilization. In the same way as a DM describes newly-discovered magical items by their appearance without giving away any details, so too must a DM describe the items and practices of civilization. From a humanoid's point of view — clothing, armor, weapons, tools, utensils — everything is strange, wondrous, frightening, and unknown. The trappings which players normally take for granted should become new and mysterious to humanoid characters.
For example, Breeka the aarakocra enters a human town for the first time. What are the strange wooden caves that humans go in and out of? Why do those humans shake hands? Or press their lips together? Or give shiny objects to one another? And why is that human yelling because Breeka ate the pig in front of his wooden cave? Breeka, who has never before encountered a human town, finds herself surrounded by unusual trappings and strange practices which she will have to spend time getting to understand. While players can roleplay a lack of understanding concerning human social customs, it is up to the DM to keep in mind that the most obvious thing to a human or demihuman is probably a mystery to the humanoid, and to describe encounter scenes accordingly.
Beyond the social disadvantages which humanoids face when dealing with communities beyond their own, there are also racial discriminations to deal with. Because most humans and demihumans see humanoids as little more than monsters, there will be extreme prejudices directed at them. Humanoids will be watched almost constantly when they enter a human community — if they are allowed to enter at all. Many towns and cities will have laws forbidding the entry of humanoids. They will be stopped at the gates, turned away, or even attacked. Humans fear that a humanoid has come to scout out the community for attack, or seeks to cause some other type of trouble. They believe that humanoids eat humans (and some do), and who wants a monster walking on the streets of town?
Many inns have rules against serving humanoids. Shops refuse to deal with them. Local authorities stay close, watching for the least sign of trouble. They have no qualms about arresting and locking up humanoids that so much as look at a human the wrong way — and banishment or confinement are the nicest things they might do to them. Mobs form quickly in the presence of humanoids, ready to take torch and pitchfork to a monster in order to protect their loved ones. Again, it is up to the DM to enforce this disadvantage. Even the most powerful humanoid PC will be hard-pressed to find a place to rest or buy supplies in a hate-filled, fearful town. If a humanoid is allowed to operate as any normal PC as far as NPCs are concerned, then a great roleplaying challenge is lost.
Another problem facing humanoids in human communities deals with the fact that things are built in human dimensions. Doors and rooms are made to accommodate human heights and widths. Chairs and beds are made to hold human weights. Even most transportation modes, such as horses and wagons, cannot sustain large-sized humanoids. This is not a problem for the man-sized humanoids, but tiny-, small-, and large-sized humanoids must learn to live in a human-sized world.