Hello
I will try to answer some questions, even if I don't fully understand them.
How is the wifi received? What do you mean by that? Not very technically: You can 'capture' the Fritzbox's WLAN and then spread it further, or you can connect the repeater to the Fritzbox with a LAN cable and then spread WLAN further. While you can be 'as far away' from the Fritzbox as you like with the LAN cable, you must of course still be within good range of the Fritzbox WLAN with the WLAN variant. Where nothing more arrives, nothing more can be transmitted. So, if the WLAN of the Fritzbox on the 3rd floor is no longer receivable, then the repeater won't work either.
What do you mean by 'socket'? If it is a LAN socket, then the answer is 'yes'. If it is a normal 230V socket, then this is a question that does not lead anywhere, see above for the answer. It is clear that this repeater does not support WLAN propagation via the mains. You would need a different product for that. If your mobile phone still has reception on the 3rd floor at a good speed, then the repeater will have the same. Possibly connect the repeater halfway.
5GHz is supported. If you can connect your mobile phone to the Fritzbox and then receive Internet via WLAN, then of course this also works with the repeater. It does the same thing as the Fritzbox, spreading WLAN. Thanks to Mesh, it's even very easy.
Is it fast? I don't know. I don't have any devices that support high speeds yet. I connected the repeater via LAN cable and my cheap mobile phone (Moto G7) shows (at 5GHz) download 280Mbit, upload 48Mbit (UPC 500/50). If I had a better phone, the download would probably be even better.