TL;DRWhat is the free subdomain provided by Google Workspace that is similar to the free xyz.onmicrosoft.com subdomains provided by Microsoft?
Let us say our business domain is www.mango.edu. We have subscribed to both Microsoft 365 for Business and Google Workspace.
Our IT administrators have probably specified MX records on both Microsoft and Google.
For now, the Google Workspace MX priority level is probably higher than the Microsoft MX record. That is, we receive and send emails on "mail.google.com". However, if we do access "outlook.office.com", we will see that it is also available. For example, my email address for Google Workspace and Microsoft 365 is both big#mango.edu. For the latter, the real subdomain email address is big#mango.onmicrosoft.com.
Example: if I (big#mango.edu) send an email to my colleague (small#mango.edu) on Gmail, s/he receives it only in Gmail. If I (big#mango.edu) send an email to my colleague (small#mango.edu) on Microsoft, s/he receives it only in Microsoft.
Microsoft 365 is not able to route emails sent to xyz#mango.edu outside of its subdomain (mango.onmicrosoft.com). That is, internal exchanged emails will only be received on Microsoft, and not Gmail. However, external users can receive my emails sent from Microsoft 365 (meaning Microsoft 365 email system works).
Question: I can make Gmail forward all emails sent to big#mango.edu to my subdomain alternative email on Microsoft (big#mango.onmicrosoft.com). This way, all internal and external emails to my big#mango.edu address will be forwarded to my preferred email address (big#mango.onmicrosoft.com also shown as big#mango.edu), and I could respond to them on Microsoft 365. However, Microsoft 365 cannot send my responses to Gmail probably due to low MX level - thus my internal colleagues can only receive my responses on Microsoft 365 and not in Gmail, and they do not check Microsoft 365 regularly. But I guess if I know their Gmail subdomain address, I can directly send my responses to their Gmail subdomain addresses (something like small#mango.googlehosted.com but also displayed as small#mango.edu).
In other words, I (big#mango.edu or big#mango.onmicrosoft.com on Microsoft) am not able to send an email to my colleague (small#mango.edu on Gmail). But if I send my email to his/her real subdomain address on Google (maybe small#mango.edu.ghs.googlehosted.com on Google), s/he will receive it.
I hope I made myself clear.