To point your domain to IONOS, log in to your IONOS account and navigate to the Domains & SSL section. Once there, click on Set up existing domain, enter your domain name, and select 1&1 IONOS Name server.
Copy the given nameservers, and add them to your domain's DNS zone.
Once you do that, DNS propagation will start. This process can take up to 24 hours. During this time, it's expected that your website won't work, but give it some time, and it will get online soon.
That’s it! Now you know how to point your domain to IONOS 😊
NOTES
If your domain is pointed elsewhere by NS records, your DNS zone is also managed from there