Questions about Radeon RX550 LP OC

I saw it too :D but as they say "If it's too good to be true, it probably is" ;-; Ber the pictures and the description unfortunately say amd :(

I assume you mean the following model: https://support.apple.com/kb/sp589?locale=de_DE Apple has actually published a list of compatible GPU upgrades for this: https://support.apple.com/de-de/HT208898. Unfortunately, this graphics card is not listed as compatible. The compatibility requirements seem to be very specific according to my research: https://de.wikipedia.org/wiki/Mac_Pro#Grafik.

Doesn't fit, from the lack of slot to the lack of space(graphics card is almost 4cm thick) to the fact that this laptop model almost can't be opened

No. The graphics card draws power directly from the PCI-E slot.