You understood everything !
To have a good note my mining plant must be profitable , I live in France so the bitcoin is not profitable but at the ETH profitable air it, I have no starting material , I think I can invest about $ 80 / $ 100 at the beginning in my little setup
$100 won't get you very far I'm afraid.
You could try to buy a second hand antminer on this forum, but as a fellow European, I can pretty much guarantee they won't make any profit (we pay to much for our electricity).
Basically, if you spend $100 on mining gear, it might take you more than a year before you mined $100 worth of btc, and if you subtract the power cost you'll make negative profit.
I guess your best shot would be to buy a second hand GPU (read up on which GPUs to buy in the altcoin mining subforum).
It will be very difficult to make profit tough