So I imported this GoogleSheet that had two columns named Theme1 and Theme1 and glideapp automatically created a column with both put together like tags (which was absolutely perfect) => See image
How can I do the same thing directly within the Glide data table? I cannot write directly in the empty cells of that column sadly
In Glide you can write all your values as a comma delimited list, and then use a Split Text column to create an array. This method is completely dynamic with an unlimited number of array items and only needs two columns.
If you instead want to maintain separate columns for each item, you can use a Make Array column to join them all into an array. But this is not as dynamic as the first option since you still need separate columns for each individual value.
As for what you see from Google sheets, Glide will create an array if you have sequentially numbered columns. The data editor will only show a single array, but the individual columns underneath are still accessible. This is a leftover feature from back when we didn’t have a data editor and things like arrays and relations needed to be built within a google sheet.
I don’t find where that split test option is 
Add a column in the data editor and select or search for Split Text.