A. Deploy a landing zone environment by using AWS Control Tower. Enroll accounts and invite existing accounts into the resulting organization in AWS Organizations.
B. Enable AWS Security Hub in all accounts to manage cross-account access. Collect findings through AWS CloudTrail to force MFA login.
C. Create transit gateways and transit gateway VPC attachments in each account. Configure appropriate route tables.
D. Set up and enable AWS IAM Identity Center (AWS Single Sign-On). Create appropriate permission sets with required MFA for existing accounts.
E. Enable AWS Control Tower in all accounts to manage routing between accounts. Collect findings through AWS CloudTrail to force MFA login.
F. Create IAM users and groups. Configure MFA for all users. Set up Amazon Cognoto user pools and Identity pools to manage access to accounts and between accounts.