Updating this takes a surprising amount of time, and keeping it current has been less of a priority than getting the mod ready for beta release. I'll make it a priority to rewrite the whole thing after we get the mod into beta. Hopefully then things will slow down and it'll be easier to keep this guide accurate. Until then, I'm leaving the old outdated version up here.

This is an alpha release. There's still quite a lot that's unfinished. We've tried to polish up what's ready, and to make sure things work reasonably well. However, there's still missing and half-finished localization. Some of these descriptions are important for gameplay, so I'll do my best to describe some of the basics and help our early players have a more enjoyable start.

Owning more planets slows your population growth. Colonizing more planets still increases your total growth, but not by as much as in vanilla. This means smaller or less expansionist empires are at *less* of a disadvantage, it also means massively fewer pops get added to the galaxy over the course of the game. Don't worry though - the way pops work in Immortal means they'll scale from being about 2x the value of vanilla pops at the start, up to about 6x the value of vanilla pops in the late-game.

Pop Sprawl Math:With only one planet, your total growth is 100%. Each additional planet adds 80% of the growth of the previous planet, as follows:

1. 100% total growth - 100% per planet - no Sprawl
   
2. +80% added growth - 180% total growth - 90% per planet - 10% Sprawl
   
3. +64% added growth - 244% total growth - 81% per planet - 19% Sprawl
   
4. +51% added growth - 295% total growth - 74% per planet - 26% Sprawl

The percentages start to get pretty fractional, but it's a relatively smooth scale up to a maximum of 85% sprawl at 30 planets.

Pops can migrate on their own. Stellaris Immortal has always had this feature, but in alpha update 3 the whole system is rebuilt from scratch to be much more performance-friendly, and solve a few other issues as well. Here's how it works:

Any pop that becomes unemployed will migrate after a random amount of time (average 90 days), provided it can find another planet with free jobs and suitable habitability.

Whenever a pop grows or is assembled, if there are no free jobs on the planet it instantly migrates, provided it can find another planet with free jobs and suitable habitability.

Trade policies have been reworked. You start the game with an alloys policy, and the option to switch to consumer goods. Once you research Planetary Unification, you'll get access to research/unity trade policy options. Even later, there's another option I'll let you discover!

Building slots are unlocked by upgrading the planet's Capital. Your empire capital planet starts with a fully-upgraded capital building. Additional colonies will take time and research to unlock all 4 building slots.

Buildings are used to specialize planetary production. You only get 3 building slots, so use them wisely. Each building provides a significant boost to certain jobs. Factories, foundries, labs, and unity buildings increase the BASE production and pollution of their corresponding jobs by 100% per tier. This means a fully upgraded lab quadruples the base science output of the planet, before % modifiers. The same is also true of Energy/Food/Mineral improvement buildings, though they only have 2 tiers.

Buildings are all planet-unique. ...

Some buildings specifically improve Custodians. Custodian jobs only come from buildings and from a special district (see below). Normally their only purpose is to remove pollution. However the Hydroponics Farms, Photovoltaic Modules, and Recycling Centers add both resource production and pollution to all Custodians. These buildings are a great way to supplement your basic resources in an emergency, but beware the pollution cost!

Housing buildings increase the planet's size. This means housing buildings are a calculated investment. These enable you to focus the planet more onto an existing specialization, but this focus comes at the cost of per-pop effectiveness that might be gained by adding another different building.

There are a variety of new "district" types available from the planetary decisions menu. These decisions each add a deposit to the planet with housing & jobs. Most of these replace former buildings that no longer fit the SI planetary development model. If you have any of these special "district " deposits on your planet, you'll also have optional decisions to remove them in case you want to reclaim the district slots.

SI replaces Amenities with Pollution. Pollution reduces pop growth and stability. If you can get it negative, you get bonus stability and growth instead.

Most jobs generate pollution. Buildings that increase job production also give a proportional increase to the pollution generated by that job.

Custodian jobs remove polluion. Certain buildings add resource production to Custodians. Careful: these buildings also add pollution production to the custodians.

Pollution colors are all out of whack! They're wrong. We know it. We tried really hard to fix it. Some of them we could, and others seem to be hardcoded. If you know how to fix the pollution color displays, we'd love to hear it! Heck, there's places where the +/- indicators are even wrong. There's not a lot we can do about it right now. Sorry. :(

All (regular) empires can build Ecumenopolises. The planetary decision is now unlocked by a rare technology that can appear once you've researched the first two housing increase techs. You can skip straight to building them earlier by choosing the Arcology Project ascension perk, which unlocks the tech and gives partial progress.

Arcologies are cheaper and easier to build. You need 4 City districts, no rural districts, and no blockers on the planet. It costs 2500 alloys and 200 influence.

Arcologies have been completely redesigned! They have 4 district types: Trade, Engineering, Academic, and one more that I'll leave for you to discover! Like city districts, each arcology provides 2 housing/jobs. If you have the Arcology Project ascension perk, each arcology district provides 1 more housing/job.

Habitats and Ringworlds are regular planets now. This means they get city/energy/mineral/food districts, and they get the same building options as regular planets. There's still a few special decisions they don't get. (no arcology-ringworlds, for now...)