B) I would say after the appropriate amount of water is present or added via asteroids/comets, whatever (and that's probably A LOT) you would first add microbes like cyanobacteria, algae, bryophytes, lichens, etc.
Next, assuming that soil is appropriate or that the mosses and lichens have leached enough acid to break rock into soil, add small plants and eventually larger plants. We could assume that these plants have been chosen, artificially selected, bioengineered, etc. to survive on whatever was lacking in the soil and atmosphere and would be more efficient at adding O2. I created astral wood, ( Populus astral) - a fast growing tree with a sapphire-green iridescent bark and slightly shimmering green-silver leaves related to poplar/aspen.
Eventually add small animal life like bugs and maybe some fungi.
Somewhere in there you might add machines that churn out O2 from CO2 or O3 or whatever is present. There are lots of artwork online that are really evocative for this atmosphere building concept.
My players PCs are mostly scientists. Early in my current campaign, they got a several month contract to monitor and analyze the progress of terraforming a world in its advanced stages. Found all kinds of stuff, like a scout wreck that crashed 105 years ago, evidence of a false pirate base that the navy planted to try and lure in criminal wannabes, the PCs even had to evacuate because of one of many scheduled ice bombardments.
Hope this gives some ideas for what you want to do.