The default authentication is Atlassian accounts, but you can also use SAML or Google Apps integration. Check out: https://confluence.atlassian.com/confeval/other-atlassian-evaluator-resources/atlassian-cloud-authentication-saml-sso-google-apps-atlassian-account-and-ldap
If you are using Atlassian account as your authentication then you have no control over the password policy and are limited what data you can see, basically just when they last logged on. If you want to be able to set password policies you would need Identity Manager or G Suite.
I briefly looked at the user agreement and there is nothing in there, and if you change your password it does not say anything about a policy, it only shows you if the password is week or strong. You also have the option to enable two-step authentication.
You may want to send Atlassian a question and ask, you can use this form for it, https://www.atlassian.com/company/contact/general-inquiries.
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