Hi @carstenmkiv,
Please try to use functions split, skip, first, etc. to extract values from the return results of a checkbox.
I have made the following test. In the form, I have a multiple answers enabled Choice column.
In the flow, the multiple answers would be displayed likes below:

Then I try to use the following functions to extract values from the results.
Add Compose action, use the following code:
split(body('Get_response_details')?['rf50b84c83d804612a77a6dff04f34645'],',')
Add Compose2, use the following code. The function first will get the first item in the collection, then use replace function to remove the special character “[”:
replace(first(outputs('Compose')),'[','')
Add Compose3, use the following code to return the second item in the collection:
first(skip(outputs('Compose'),1))
Add Compose4, use the following code to get the last item in the collection:
replace(first(skip(outputs('Compose'),2)),']','')
With all the separate items returned, you could enter each item to corresponding column.


Hope it could be your reference.
Best regards,
Mabel Mao