I’ve found out something really interesting when playing the Sims 4 is that there is 2 types of jobs. One that you can sign up to via phone and another one is auto assigned to NPC.
While one member of one of my household was “flirting” with a sim on the streets and found out she was a mail courier person and invited her to a bar and talked more.
Then the next day I was ready to meet her again at my house and noticed she rocked up angry and was wondering what happened that pissed her off so I decided to add her as a family to find out. Of course one of her personality was hot headed sim, so she was always angry and likes to talk to herself. Then when I moved those 2 sims out to a new lot, I’ve noticed she’s no longer works at her mail courier service job and was instead unemployed. Then I started digging and after she became a supermarket shelf stacker.
Another time where I had a big house and decided to get some roommates to stay within the lot and after a while I manage to convince a gym trainer that I couched me in the past and build a relationship to stay as a roommate. Then later on I’ve noticed a bookmark icon appeared as the bond between the 2 sims became closer. Then I decided that rather then keep as NPC and helping around the house (which she was good at compared to the other sim), I decided to add her as family. Then I noticed the gym trainer job was no where to be found and the sims ultimately went unemployed.
After doing more studies, I’ve found a Tumblr page called Beth’s Sims and explains those jobs are meant for NPCs and not available for regular households that are being played by the player. Which explains why when I add sims as family they no longer have their job active unless they are doing a job like criminology and also hidden NPC jobs from each expansion packs.
I’ve tried above command from Beth’s Sims to assign gym trainer for one of my sim, it seems that didn’t work so I tried (just for fun) without the ID being entered and it seems that I’ve got the job as a gym trainer. However, the job doesn’t have a place assigned and also there is no real progression like real jobs with completing daily tasks and there is no days off or holidays for that job or any NPC jobs for that matter.
Update: It seems that jobs are not active anymore when you exit/reload the game/household. Mailman and gym trainer are none assignable. As for gym trainer, it auto assigns Level 10 fitness NPCs “at times” (not sure what are the chances of assigning one of your NPC creations into a gym).
However there is a mod called NPCC (short for NPC Control) which allows you to assign NPC jobs to unemployed sims as shown in one of their examples. However, the sim must have their skills maxed out in order to assign those jobs like the gardener with level 10 gardening skill.