1) To set up correct DNS records in your Google Domains first, log in to your Google Domains account.
2) Then find the menu "My domains" where you must go to the tab "Manage".
3) After going to the "Domain overview" page, you should go to the "DNS" section.
4) When you get to the "DNS" section, you should ensure that all your DNS records are in the "Custom records" window (this ensures good operability and the ability to change your DNS records). Usually, Google sets up needed records in the “Default section”. You need to copy MX records to the “Custom records” and then delete “Default records”. The issue with them is that they are not 100% correct so you need to create polished new records in the “Custom section” instead.
If there are no changes in the DNS records authentication after you made all previous steps, check out the additional information below.
Important! If you use third-party sites with the ability to administer DNS, such as Wix for example, make sure that the NS record of your domain matches exactly on Google Domains.
(What is the NS record of your domain you can check here. Google Domains NS records are most likely like this example: NS-CLOUD-B1.GOOGLEDOMAINS.COM)
If you want to set your own custom name servers, then you can add them in the "Custom name servers" tab.