General questions about DNS / Domain Management [How to connect a domain with lightspeed]

Hi all,
I hope my question is not too stupid- But I'm struggling with this since some days.
My question is, how I properly connect a Domain from [VENDOR A] with my VM running on [VENDOR B].

In my case- vendor A is Strato. I'v bought a domain from them - Let's call the domain:

Afterwards I've created a WordPress VM with Lightspeed form my [VENDOR B] (e.g. Google Cloud,, AWS..)
The VM is up and running - I've told the SSH init script :

Please input a valid domain:
Please verify it is correct. [y/N] y
Do you wish to issue a Let's encrypt certificate for this domain? [y/N] N
(Would like to do this later - but also here - no idea yet how to connect my domain to the certificate.)

However, I've no clue what's next?
How do I tell Strato the right DNS-Server to my Server? - Sure I could use the in build DNS features from e.g. digital oceans- But there must be another way.
Do I need to do further configurations on the server? (It's Ubuntu 18.04)

Thank you,