In my experience, there are generally four stages to sex: enticement, foreplay, intercourse, and cool down. These are very broad categories which encompass a lot of different things, and often are very different for each person. Also, you might go back and forth between these activities throughout the encounter. But for the majority of sex, this is the overall pattern that occurs.
To illustrate what happens, I’m going to use the example of Alex (male) and Libby (female). They met on a dating website for casual hookups and decided to meet up at a coffee shop, and possibly more if things go well.
Alex and Libby meet and chat for a bit; they are attracted to each other. Their conversation becomes increasingly flirtatious. Alex might test Libby by lightly touching her arm to gauge her reaction, while Libby smiles a lot and plays with her hair. They both get aroused at the idea of a wild night together as they talk and flirt. Finally Libby musters enough courage to suggest going back to her apartment, and with clear consent from Alex they head out. Once they arrive at Libby’s place, they sit on her couch and have a drink together to relax and get comfortable with each other.
Libby leans in and kisses Alex, initiating foreplay. This includes making out, removing clothes, touching each other, oral sex, fingering, toe sucking, handjob, etc.
Alex becomes so aroused that he decides to progress to the next step. He asks Libby if he can fuck her. She gives clear consent, so he puts on a condom and slowly puts his penis into her vagina, ensuring he is not hurting her and that she’s lubricated well. He may also put his penis into her anus, if that has been discussed and pre-approved and again she gives clear consent to do so. They switch positions every few minutes to distribute the workload; when Alex’s legs get sore, he lies down so Libby can take charge by being on top, and when Libby slows down she gets on her knees so Alex can penetrate her in doggy style. After a while, Alex may decide that Libby isn’t wet enough so he may switch back to foreplay by giving her oral sex. Or maybe when they switch positions, Libby surprises Alex by giving him oral sex. Perhaps one or both of them cannot orgasm from penetration and must revert back to foreplay to finish. Ideally when both parties are fully satisfied, the sex is finished.
Cooling down can include clean-up, a drink of water, exchanging phone numbers, discussing the good/bad parts of sex, or just lying there comatose while staring at the ceiling. It’s whatever happens after sex, whether it lasts for 5 minutes until Alex quickly leaves, if it’s just a break before round two, or if both Alex and Libby fall asleep entangled in each other’s arms.
Keep in mind this is just one example. The events would likely be different if Alex and Libby were dating, or if they were having sex in public, or if there were other people involved (like in group sex). The process is generally the same with those four stages, but the way they play out can vary a lot.