Can someone else access my iCloud ?
Some stole my phone and ever sense it was returned it hasn’t been the same ...
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Only those who know your AppleID password and have access to one of your trusted devices can do so.
Make sure you have another method of receiving auth codes like another trusted friend’s mobile
https://support.apple.com/en-us/HT204915
You can log into icloud.com and make sure any unknown devices are removed from your account.
https://support.apple.com/en-us/HT204074
More important if you are concerned change your appleID password.