Currently, there is no way to get through API the Teams that a user is a member of, or the members of a Team, such as is possible with groups in:

• https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-users/#api-rest-api-3-user-get
• https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-users/#api-rest-api-3-user-groups-get
• https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-groups/#api-rest-api-3-group-member-get