Retainer venture question

[Felis]

I thought when my DOL become Level 50, I can equip them with my 1 star items I use for my normal DOL. But everything I hand over to them is gone from my armoury. Is that how it is supposed to be?

Yes. this is how it is supposed to be. The moment you give a retainer an item (in his inventory, bazaar or equip window) your character don't have it anymore with him until he take it again from the retainer.