In a view in D8, I want to achive something like :
term 1 -item 1 -item2 term 2 -item 3 -item 4 term 3 - item 5 - item 6
Is it possible ? If yes, how ?
It sounds like you are looking for a glossary view. There are many tutorials on how to do this, ex: https://www.ostraining.com/blog/drupal/glossary/
... after rereading, you may actually be looking to create a view of all terms in that is grouped by parent id. There are different ways to do this. Take a look at:
These talk about D7, but the same should work in D8