How does Heat work anyway?
Every region of the world you work in has an independent Heat rating that increases long-term whether you do anything or not (you can slow this via Research and the Control Room) and increases quickly when you do Schemes. There are also Heat reduction Schemes that help you drop it. You could also wait for the Heat to become Maximum, triggering a 5 Minute lockdown of your Criminal Network in the region and resetting to 0 afterwards.
Every Faction like “P.A.T.R.I.O.T.” (Blue Torch); “S.M.A.S.H.” (Yellow Symbol); “H.A.M.M.E.R.” (Red Hammer); “A.N.V.I.L.” (Orange Anvil) and “S.A.B.R.E. (Green Sabers) has a Heat rating that adds all the Heat you have accumulated in their regions to a total number. So, say you have four Lvl2 Operations running in “H.A.M.M.E.R.” territory, the faction Heat can add up all of that to a Maximum of 400 (100 from each region).
You can see your current Heat Level for a Faction added up at the Top right of the Screen via the “F.O.J. (Forces of Justice) Heat Tracker” or in the Stats Screen, “System Menu” bottom left > “Stats”. There is also a “F.O.J. Threat Level” indicator under “Stats” > “Global Operations” showing when to expect dealing with heavier resistance:
Usually the Faction you have the highest Heat Level with at the moment will send out Agents to your base next. It determines what kind (Type: Investigator/Soldier/etc.), what amount and what Level of Agents they send. Before the timer runs out you can still decrease the Heat. Once it runs out it’s determined and they’re on their way. They’ll arrive either with the next boat or via speedboat right away and you’ll have to deal with them in your base. I believe it’s more likely for them to send Soldiers if you kill or capture a lot of Agents.