What needs to be considered when buying a notebook, so it is not slow and I can use it for everything?

For example, memory, etc.

If you want to do yourself a favor, avoid the following:

Intel Atom, Celeron, Pentium Nxxx / Silver and Gold.

AMD A4-12.

Avoid if possible Intel graphics, if you are only doing Office, YT oä.

Usable are from Intel i3 and AMD Ryzen.

…

GPU Then as minimum APU Ryzen 2200u with Vega 3 graphics, then only lowGames.

Next lowGames GPUs: Nvidia 130/1030, AMD RX550, AMD Vega 8

lower median: Nvidia 1050

better: Nvidia 1060

upper class Nvidia 1070/1080

CPU as mentioned above from Intel i3 and AMD Ryzen 3/5.

SSD from 120GB absolute minimum, better 250GB

RAM from 4GB upwards

For 1200 euro you get something decent.

It depends, if you do not play, then just a small notebook with SSD and IPS Full-HD Panel, that's enough! Then there should be a simple Pentium or better, at AMD an AMD Ryzen, which does not matter, the main thing is not an AMD A processor.

Without SSD, your notebook is actually useless