To know if there are direct trains or not, please check the timetable, such as on the DB website or ÖBB website. The timetable in the Interrail app is not reliable.

I'm not sure where you saw "8 days travel included”. Do you have an URL?

I suppose it's one of the flexi passes that includes X travel days within a Y period.

Immediately after choosing the dates and places it asks you how many travel days you want to do, I did not understand what is meant by "travel days".  And another thing, is it valid both you go and return?  Because I would like to go all road trip by train and return by plane, to be faster

A travel day is a day on which you travel. You can then use as many trains as you want in the area where your Interrail is valid. Also see:

https://www.interrail.eu/en/support/interested-in-interrailing/what-is-a-travel-day

So yes, you can make a return trip if you want.

So it's only worth the one way?  Do I have to organize another trip for the return?

Interrail is valid for any direction so you can also use it to go back to your home town.