The closest understanding I have to this is that pilots follow “sky highways,” but how do pilots know they’re still in their assigned highway? Doppler? GPS? Practice? Cloud magic?
PS please pardon me if the flair isn’t correct, I wasn’t sure what to file it under
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In addition to the other answers, I’ll add in a bit more information from a somewhat “technical” perspective… maybe closer to ELI10 level.
When flying, pilots are governed under one of two flight rules:
* Visual Flight Rules (**VFR**)
* Instrument Flight Rules (**IFR**)
When flying under **Visual Flight Rules**, the pilot has the primary responsibility to “see and avoid” other aircraft — mostly by looking outside, scanning for other traffic, and using the radio to self-announce their location (and listen to other pilots announcing their positions).
Pilots flying VFR have a lot of freedom to fly where they want to go, and don’t have to follow the “sky highways” (typically, [Victor airways](https://en.wikipedia.org/wiki/Victor_airways) in the US & Canada). Sometimes VFR pilots navigate by simply flying above local roads, Interstate highways, heading towards known landmarks (big lakes, stadiums, etc.), or even just fly a compass direction (using [dead reckoning](https://en.wikipedia.org/wiki/Dead_reckoning)).
Since they may not follow any airways, and don’t have to fly according to any set flight plan, it is their responsibility to visually see where they’re going and not collide with other planes. Nowadays there are [systems such as ADS-B](https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillance%E2%80%93Broadcast) that can map the positions of nearby aircraft, but not all planes have them so by and large the pilots need to literally look out of the window.
When flying under **Instrument Flight Rules**, the pilot must file an [IFR flight plan](https://en.wikipedia.org/wiki/Flight_plan) and coordinate the flight with Air Traffic Controllers (ATC). The flight must then precisely follow the flight plan and be conducted under [ATC clearance](https://skybrary.aero/articles/atc-clearance). ATC will then ensure that IFR aircraft remain safely clear from other aircraft within [controlled airspace](https://en.wikipedia.org/wiki/Controlled_airspace).
These days, there are many aids to air navigation used in IFR flying — including GPS and collision avoidance systems like [TCAS](https://en.wikipedia.org/wiki/Traffic_collision_avoidance_system) — but strictly speaking IFR planes don’t crash into each other because they all fly under ATC control and [ATC has the responsibility to provide separation](https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap5_section_5.html) between them.
TL;DR: pilots can fly under VFR or IFR rules. VFR pilots must “see and avoid” other airplanes, while under IFR the Air Traffic Controller is responsible to keep airplanes clear from each other.
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