nested views of main CT and sub CT What I want to achieve is nested view.

Say I have Content Type 'main' en Content Type 'sub'

How can I create a view that looks like this:

main 1 title
main 1 fields
    sub 1 title
    sub 1 fields

    sub 2 title
    sub 2 fields

    ...

main 2 title
main 2 fields
    sub 4 title
    sub 4 fields

    sub 5 title
    sub 5 fields 

    ...

...


Do I need references?

more context:
I know there is something like entity-reference and also the use of tags.
For every sub I placed 'main 1', 'main 2' or 'main 3' as a tag.
I also let every sub refer to content 'main 1' , 'main 2' or 'main 3' with the entity reference (set on main)
I tried already by creating a view of 'main' and a block view for each 'sub' with tag as extra filter. I don't know though how to make the view 'main' implement each 'sub' view and place it in each main node.
Is this clear or do you need more info? If so, which ?
Have a look at the Views Accordion module, and its related Community documentation.

By "grouping fields", the group header can be used as accordion header. And Views allows for one field to include the content of multiple fields, so you can utilize this views feature for this purpose (Combined fields).

Does this help somehow?