Suppose I signed up in a website which creates a bitcoin wallet address for my account and provides me with this address (something like 2RfG1Ph5KlojhnFi8Dg36lfeshnfDiL3Ji) and the QR code.
No.
If this were possible I could simply create a wallet from that information and move the bitcoin around as I pleased.
That's simply the public address to which bitcoin can be sent.
There's a separate mechanism or entry point for sending bitcoin. With electrum for example, as it's a deterministic wallet, the private key is determined by the seed. The seed here being a sequence of words.
Entering those words into Electrum acts as a significantly difficult password. From that seed the private key is generated. It's the private key which enables you to send bitcoin.
Loss of the seed words and loss of the private key effectively result in loss of the bitcoin at that address. (Until Google cracks the whole thing with quantum computing, of course.)
