Correct that resistivity in given table is the DC resistivity in https://mycableengineering.com/knowledge-base/conductor-resistance ?

Should it be Ω/m as resistivity is proportional to length?  

How to calculate α20 when temperature in cable is changing? Possible to assumed it's constant in temperature range og 20-90 degrees celcius?

The values are the ones you should use for d.c. resistivity and the units are Ω.m.  

For 20 to 90°C, normally people would assume α is constant.  Using a more accurate value is probably not going to make much difference (unless you are doing laboratory type experiments).

If you are interested in how α varies with temperature, an Internet search will give more information.

Great, thank you!