Automaticas

I count from the time they pop their heads above ground. Some start 7 days after they come up from soil. Either is acceptable. The first 7 really has no progress and can be considered seedling. Once the first set of true jagged edge leaves show "normally 7 days from popping up" then start week 1 veg. Goodluck growmie.

You're at the right time to give nutrients when the first two t4 leaves have unfolded, so just start SLOWLY WITH ROOT JUICE AND SUCKERS AND GROW OR BAT 🦇 SHIT 💩 IT WORKS. HAPPY GROWING!

You start counting the days when the first set of true leaves appear.

Never. CO₂ bags are scam to me. Get a good light, good nutrients, take care of your plants and when you've reached the maximum you can do, then you can start thinking about adding CO₂. And if you do, use bottle and electrovalve in sync with your fan and extraction system.

Its kinda pointless to use CO2 unless your grow room is sealed up, otherwise its just going to be venting out, and if you are venting out into your living space higher CO2 is not ideal. You can use CO2 at any point in your grow. if you only have a bit of it, I'd use it during the flowering period, If your other environmentals are in check the plant will drink more and can take more light while the CO2 is in use, but this is not really something for beginners. I've never used CO2 in my grows and I don't think I ever will. just seems pointless for me as I do not have a sealed tent. Good Luck!