Seeing Google Workspace and/or Microsoft Office 365 in the list of senders in your reports is normal and is something we see in nearly all of our customers' accounts. As they are two of the biggest corporate mailbox providers, a lot of your emails will end up at one of those two providers as a final destination. These users may have auto-forwarding rules set up to automatically forward any emails they receive to another destination.
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For example, assume that Mimecast is your sending server:

Mimecast → Google Workspace or Microsoft 365 (auto-forward) → Hotmail (or other third party server)
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Because DMARC is based on the last hop of an email, when the above happens, Hotmail will see Google Workspace as the sender of your emails. Therefore, if you are not using Google Workspace or Microsoft 365 but they still appear as a sender in your account, it is because they are forwarders and there is nothing to worry about.
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NOTE: In the above example, if Mimecast was only configured with SPF then emails would fail DMARC when auto-forwarded since SPF breaks during forwarding.
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It is therefore imperative that your sending sources be configured with both SPF and DKIM when at all possible. DKIM signatures travel along with your emails when auto-forwarded and also form the base of ARC, which is why DMARC will pass, and the final recipient will be able to validate that they have come from your legitimate source.