What Zubas said.

Scroll to the bottom of your "Available Projects" list and there will be options like "Economic Stimulus", "Cultural Festival" - those will run forever once you pick them.  You can get other options via tech that'll boost ship building, research, etc.

There are a few things to note about these.

If your planet gains new tiles via events/affinity/tech, the planet will still be doing that continuous project, so you need to go deal with those manually.

If you start any other type of project, like say there is a new tile and you have ti build a farm there, the game will auto place that before the ongoing project but won't terminate the ongoing project - which is something you may want to do in some situations (ex, you research a terraforming type of tech, and you set that up to open up a new tiles, you'd want to put a new project on that new open tile when it's done, without "losing" it to the ongoing project (if the ongoing project is still there, you won't get the planet idle event).

If you research a tech that upgrades tiles, that will automatically happen - the upgrade projects will be placed ahead of the ongoing project.