In the "olden days" before digital movie projection and the Digital Cinema Package were around, movies were recorded onto film (usually 35mm, I have heard of 70mm instances). The "master film" was the 35mm roll, and somehow copies were made to be distributed to theaters, which could also be in the 35mm format but 16mm could be used as well. Just how was this done effectively?
Regardless of format; 16, 35, 65, etc... The studios would distrubute release prints. The prints were shipped (speaking about 35mm here) on 2000' reels; 5 reels to a can.
In the olden day's they would use two projectors and the projectionist would have to literally load reel one, and two to the other, and they would switch over. Back and forth. A typical movie in today's world (2 hours) would run about 5 reels.
So let's talk the 1990s. The theater receives a can of 5 reels. The reels would be sent up to the projectionist booth. The projectionist would then assemble the reels into one giant reel; on a platter; basically a large spinning lazy-susan. The platter can feed either from the inside; called the core (the core is roughly 10 inches; and has a center mechanism so with rollers in an assembly so the film never has to be "rewound".
The projectionist would first add blank leader, just black, typically 30 seconds; then they would use a film splicer and film tape to add the theater opening title, and each trailer which would play before the film. Some films; shipped with specific trailers on them. But most times; there would be racks and racks of trailers (small film reels about 6 inches (3 minutes) of film; and the projectionist would; whatever his preference or theater policy; add the trailers he wanted (typically matching action with action films, etc). Then any other small film reels would be added; "Turn off your cell phones". Etc.
Lastly; the projectionist would add reel 1, then 2, and so on; and the platter is motorized so it literally pulls the film off the reel which is setup on a rolling stand.
Each reel would need to be checked at the tails; and spliced and glued together.
The final reel would be a very large reel of film; roughly 5 feet in diameter, with a core about 10 inches in the middle.
Because this would be the "first run", the start of the movie would be on the inside, in the core; so the film would be pulled out of the core; through multiple rollers; and into the projector; and then fed to a take up platter either below or above the feed platter. (Typically there would be 3 platters so you could have more than one movie available on any given screen).
This setup would yield after the movie with the film left on a new platter, ready to be "re-threaded" through the projector. Every showing required the projectionist to check the platter; and reload the projector threading the film over rollers with sprockets; through the gate, and then to the take up reel.
Release Prints were VERY expensive for the studios to produce. So many theaters had rollers for "Big Releases" - think "Titanic"; where one platter would run through the projector, then instead of going straight to a take up platter; it would be fed through a series of rollers down a long wall; to a 2nd or even 3rd projector; and they would have 3 screens starting all at the same time. The final take up reel would be at the last projector.
After the film was finished being run. The projectionist would then literally do the opposite of putting it together. Trailers typically discarded; and the film put back onto the 2000' reels. The film reels would be put back into the can; labeled for FedEx, and sent to a B-Theater; a less popular/lower income movie house.
This is why when you saw movies in the pre-2000s at say an old mall movie theater; there would be lots of scratches and dust, because the film had been played so many times and had been out for so long.
Now; you can find release prints on ebay; I almost bought a "Terminator 2" release print just to have... they were owned by the studio, and destroyed after use... so they are fairly scarce.
The projectionist would have to pay close attention during assembly and tear down; because you cannot rewind the platter.
I speak from being a projectionist for a year after highschool. One time I assembled a movie in the order of reel 1, 2, 4, 3, 5. So half way through the movie people got confused and came out and complained. With no way to rewind or fixed, the rest of the days shows had to be canceled, and the film reels taken back apart and put back into the same order.
Last note; you can typically see the change from reel to reel when watching a movie; either by seeing what's called a cigarette burn (small circle top right of frame), or simply, you notice a weird cut/shift in color/sound where the splice was... Because the release prints were made on multiple printers, color always didn't match reel to reel. Most times, you could at the least (if the film had an optical audio track); a small pop. Towards the later 90s... films went to digital DTS, and THX went digital as well. But before that, the film was a waveform optically printed on the left side of the frame. The audio, was optical.
In terms of how they did it "effectively". It's just how it was done for almost a 100 years. They were very very happy when things went digital. And theaters had to literally fight the studios, because most projectors in film houses were decades old... mechanical... and the studios wanted them all to upgrade to 20K Lumen 4K Dual Projectors (Digital); which run about $200,000 each.
It would have been unusual for 16mm prints to be struck from 35mm negatives. This would only be done with an optical printer, which is an expensive frame-by-frame process. If an extensive 16mm release were needed for a 35mm film, a 16mm interpositive and duplicate negative ("IP/dupe neg") would be made using an optical printer and distribution prints struck from that. Many dupe negs could be made from one IP. It would be more common for 65mm (so-called 70mm) films to be treated this way using 35mm dupe negs.
Typically 35mm prints were struck using a contact printer, a continuous-flow process, from one of a set a daughter negatives. These second-generation negatives were made using the IP/Dupe Neg process mentioned above.
In the early days prints would be "bicycled" from theater to theater (town to town), with second run and third run venues getting the prints in order. By the end of the chain the prints would be in very poor condition.
Of course, these days the master negative isn't nearly as valuable because everything is done using a digital intermediate process, where the true master is bits on a server that can be "printed" at will.