Help with Mysterious iCloud Email on My Account and Apple ID Management

I recently bought an iPhone and tried logging in with my existing Apple account, which I created a long time ago. When I checked the "Sign-In and Security" section, I noticed that my account had a primary email ID and also an unfamiliar iCloud email address (xxxxx@icloud.com). I have no idea how that iCloud email got linked to my account, as I never created it. I contacted Apple Support about this, and they mentioned that Apple cannot delete the iCloud email. This has made me feel like my account might be compromised. Additionally, I asked Apple Support if they could remove my primary Apple ID (which is a Gmail address) so that I can create a new Apple account with the same Gmail address. I'm concerned about the security of my account and don't want any unknown Apple ID associated with it. Can someone kindly help me resolve this issue?