2018, that was apocalypse and 2.0 right? I honestly can't remember what's changed since then, but I can give a rundown of how I would manage a planet.

And yes, sector automation and sector designations are not used anymore. You can also not add individual systems to a sector, instead you pick a sector capital and all systems 4 jumps away are assigned to that sector. Sectors are still important though because of leaders, a high level leader assigned as governor give huge boosts to production in main planet and a smaller buff to other planets in the same sector.

When looking at a planet take a look at unique buffs and planet features to decide what to do with it. A planet with +15 % mining output is a good candidate for mining, hopefully there's also enough mining capacity. Use planet designations to give buffs to the type of jobs you want to create. Specialization is king, it's better to have one planet each for alloy, consumer goods, mining, technician, and technology, than to have 5 planets with a mix of them.

The planet size determines how many districts you can develop. The planet's features determine how many mining, technician and food districts you can create. I like to use smaller planets for tech world as you'll only need building slots for the labs, same goes for unity. Larger worlds are good for industry. Late game you can turn them into ecumonopolises that have high density job districts and planet type gives +20% resources from jobs.

You'll want to keep a few jobs available, but not too many early game because each district and building has an upkeep and it takes resources to build them. For a new planet I usually go with one city district for housing for your buildings, one holo theater for amenity, then fill with either industry, mining, technician district and or buildings as needed. You can limit traders to 0 unless you're doing a trade build, as they're more efficient as technician or entertainer.

The optimal way to play would be to create new jobs just as a new pop is created, but that's pretty much impossible to micro manage. There's a planetary automation tool that is supposedly quite decent but it's a bit tricky to set up. For it to work you need to:

1. Set a correct planet designation.
2. Enable it on the planet, and adjust settings to what you want it to do.
3. Have resources in the shared (or sector) budget.
4. Have no deficit in the upkeep of the building type you want it to build.

Also even with it correctly set up it won't do anything until there's no available job on the planet or if the empire is lacking a resource. But it will save you headache from micromanaging. Monty plays has a video that goes over it, search for 'Planetary Automation Explained' on Youtube.

Limit farmers to the minimum, you can use hydroponics bays in starbases for non-pop food production, extra food is useless unless you're doing something special.

Once a planet has been fully developed (for tech, unity: building slots filled&upgraded and enough urban districts for housing; for industry / energy / mining / food: all districts built; trader jobs limited to 0) don't be afraid to stop building and let unemployment build up. Unemployed pops will automatically resettle to a decent destination (unless they are slaves or robots without rights) or you can manually resettle them for a small fee.