If I want to buy CHF 200.- worth of BTC, I can subscribe to Binance and there I would receive 0.003665 BTC (as of the time of writing). I would guess that this would be already the net amount, with all fees detracted - correct?
That's hard to say. You will have to check what exchange rate Binance gives you when it's time to pay. The rate changes often due to bitcoins volatility and price that rises and falls every second.
You can check how much you will approximately receive here >
https://www.binance.com/en/buy-sell-crypto?channel=hzBankcard&fiat=CHFSo, when I have that amount of BTC, it is not yet in my wallet to be spent - correct? In fact, it is still deposited at Binance.
When you have transferred your fiat money to Binance and the transaction has been confirmed, they will send you the coins to your registered Binance account. After that, you are free to withdraw the bitcoin to your Electrum wallet.
Afterwards I decide I want to use BTC to pay something I buy online. That's where my wallet comes into play, right?
If you want to purchase something with bitcoin, you can send the coins directly from your Binance wallet. There is no need to withdraw to your Electrum and then pay from Electrum for whatever it is you want to purchase. Doing it that way, you will have to pay Binance their share of fees + miner fees for the second transaction that goes from your Electrum wallet to the site/person you are paying for the goods/services. This only applies if you intend to spend the entire sum that you purchased on Binance on the stuff you want to buy. If you are spending just a part of it, it would be better you withdraw the entire amount to Electum first. Because each withdrawal from Binance will cost you 0.0005 BTC in fees + there is a minimum withdrawal limit you have to respect as well.
I go to Binance and withdraw some (or all) BTC. At this point, I have to pay 0.0005 BTC fees, which would leave me with 0.003165 in my wallet - right?
If you have 0.003665 BTC, minus the Binance withdrawal fees, you'll be left with 0.003165 BTC after the withdrawal. That is correct. Pay attention to my previous reply regarding the withdrawal fees and the way you intend to spend your money.