About modular in UE4

Hi guys, I hope you have a nice weekend.
I’m still learning UE4 and I have to clear some questions in my mind.

At the moment, I’m creating some modulars pieces than import into UE4.
As I know that I have to finished everything before import to UE4 ( Model, UV, Textures, Materials…). Because after imported these assets into UE4 I won’t able to to edit them inside UE4 (Like edit Model, UV,…). So i have to done everything all of these in external 3d package.

Please let’s me know if i wrong because i’m not a dev/ programing guys sorry about that.
Thanks for your help.