they do so...?

"so", in this case, means the action mentioned in the previous sentence.

For this usage of "so", here is an example:

A: They were killed in the accident.
B: I have already said SO.

"so" in this case refers to the thing that mentioned in the pervious conversation, which means "they were killed in the accident".

To make it clear, if we don't use the word "so", we should say:

A: They were killed in the accident.
B: I have already said that they have been killed in the accident.

I guess that your sentense is from a biology textbook talking about plants or something, I give you one more example:

"Plants don't produce carbon dioxide during photosynthesis. Instead, they do SO in a process called respiration."

That means "plants produce CO2 during respiration."

"they do so" : 佢地咁樣做