Artifact heat affects the amount of progress you get from each artifact towards a new booster. At 100% heat you will gain 0.15 towards a new booster at 1.0. The lower the heat the less you will gain, for example at 50% heat you will only gain 0.075 towards a new booster and .03 at 20% heat.

Each level will start at 100% and will drop by 1% for each artifact collected regardless of type, artifact heat will never drop below 20%. To increase heat you will need to collect 3 artifacts in another level, this will increase heat by 1% in every other level you are not playing.

The amount of artifacts in a level is fixed, which means the same amount of artifacts in a level will be the same every single time you play the level, this is true for zones as well. If zone_40 has 3 muted 2 bold and 1 aggressive artifact then it will have 3 muted 2 bold and 1 aggressive artifact every single time, artifact heat has no impact on spawns.

You can find a spread sheet with all of the level's artifacts as well as data used for testing the artifact heat here! https://1drv.ms/x/s!Anuv-Aaf-XBsg7hIID0m0T0iLDqr2A