That's the tricky part. You don't have to declare your BTC in the first place when you buy it. You have to declare it when you buy something with it in € or when you cash out in €. When they switch from crypto asset to fiat currency €. You don't have to do anything if crypto stays in stable coins or if you do crypto trades like BTC/ETH but you need to have a "Book" of everything trade to be able to calculate the plus value or minus value. If your gains are more than 367€/year, you pay 30% on the plus value. 

There is a grey zone on the buying part. If you buy BTC on a custodial websites like coinbase, binance, etc. the French gov is considering you are opening a new bank accounts on a foreign country. The same thing I need to do if I open a € bank account in Switzerland for instance. The goal is to avoid money laundering. If you dont declare a crypto bank account it's a penalty of 750€ PER account. French gov pushed Binance to open a French company to force them to follow French regulations and obey when the French economic gov ("Bercy") will requests trades from all the French People. 

If you use Robosats or Peach to buy BTC on a non custodial wallet, they won't see and know anything but techically you should declare it. 

50% at least of the People I know didn't declare anything for years because it's complicated. I'm 100% convinced that French gov (that is in complete bankrupt) will organize in few years an enormous racket on all the French crypto players and ask them for tax because they need money to finance their incompetance. Welcome to 🇨🇵😅