I have started looking for data about economic activities for construction sector, but I ended up founding companies that may be relevant actors in the flow diagram of each material, so I kept collecting these rough information at company/actor level, but I don't know if I have collcted and stored those in a correct way....

I see now that you added them one by one. It depends on what you want to do with the info.
For starters, it might be nice to just group them in different lists, one for extraction, one for construction, one for waste treatment etc. You can use the template that we've provided here, where in the comments column you could also add the URL to their website and then you tag them with actors. Then you can upload them as GPS Coordinates item type and this way create a map with their location. (We will show you how to process this next week in the course.)

Based on that list, you can then try to find their flow data: how much do they extract, use, waste etc.

Does that help?