Depends on the setting, I should think. Even within D&D proper, different settings have different rules, e.g. Eberron (IIRC) has demons bound under the world, the result of Eberron trapping Khyber beneath her body for Khyber's murder of their brother, Siberys.
In my Jewel of the Desert setting, there are three ways they can "enter" the world to some degree.
1. Very powerful (we're talking balors and the like) demons and devils can personally travel to the mortal world, though this is costly to them--they'll be weaker than they would otherwise be. They can also use this same power to send their servants, who won't be weakened as much but will still be weakened.
2. Mid-level fiends (and anything more powerful) can project into the mortal world under certain circumstances, usually related to symbolic activity on the part of mortals (symbols and symbolism have objective, tangible power to outsiders like fiends and celestials.) These projections are lesser, more like reflections, but they come with added safety (see below).
3. Any fiend, regardless of power, can be summoned into the mortal world by a spell of appropriate power, usually accompanied by some kind of offering. This is mostly done by Waziri mages or particularly reckless Sha'iri (=sorcerers), as neither Kahina (=Druids and Shaman) nor Safiqi priests (=Clerics)/Temple Knights (=Paladins) have any real motive to call on fiends for help or guidance.*
Properly killing any outsider is a challenge, as they'll technically just discorporate. Discorporation is very painful, and significantly weakens them--meaning their rivals or enemies could kill them for good in their weakened state, unless they can re-corporate in a safe place. But if you can trap a fiend (or any outsider) that has bodily entered the world, in a place where planar travel is cut off entirely, then when you kill them they do, in fact, die for real. The party has successfully killed one greater succubus this way, and that's one (of several) reasons why powerful fiends rarely choose to travel bodily into the world. It's not worth the risk; sending servants bodily, or projections, gets most of what they want without the cost.
Beyond that, devils make pacts and demons make deals. A "pact" is a formal agreement, which both parties are magically bound to obey. A "deal" is a completely informal, often freewheeling agreement that both parties can reneg upon, though a demon that gets a reputation for breaking deals is going to find their potential "customers" demanding rather more payment up front, so to speak. Most demons don't really bother with dealmaking anyway though. Devils love pacts--and, importantly, they specifically design their pacts to be things that both sides want to complete. Getting your soul is, almost always, a lame consolation prize compared to whatever they've asked you to do on their behalf.
*Kahina generally draw on spirits, which are not outsiders--this is why Safiqi banishment magic doesn't work properly on them, nor on the spirits of the dead. Safiqi do not generally summon anything, though they claim (and the party has found reason to believe) that celestials once walked this world and gave guidance to its denizens. Tenryu Shen, a gold dragon from the faraway land of Yuxia, the Jade Home, is functionally part-celestial and thus afforded immense respect by the Safiqi priesthood despite being a foreigner with somewhat divergent beliefs.