How to access AWS resources created by secondary root account?

Background: A year and a half ago my company purchased a web development project from a software company. Now I try to access the recourses in the AWS console, but I can't see any of the created and running resources. E.g. I see no active EC2's using the management account. I only see that the resources are being billed from the cost management, including EC2.

I created an AWS account (root user) and added the software company's personnel as parallel root users (AWS oranization's structure, where my account is the management account). I don't have access to the other created root accounts.

Question: How can I access the resources created by other accounts in the organization? I would like to change the git repo where EC2 is pulling the code it runs etc.

I am familiar with Azure, but AWS has always been very unfamiliar to me.

I tried to familiarize myself with the access policies (IAM) of AWS as well as search for best practices on how this is done. However, I found that creating parallel root users is not definitely the best practice. Thus I did not find any resolution to the initial problem.

I tried to use AWS Resource Manager, to find the resources, but it does not seem to help as the resources created won't show up in my management account.

I tried to use AWS Resource Explorer, to find the resources, but it did not show the active resources either.

If you have no access to the account included in your organization and from where the billings are generated, I guess that the only way is to open a support ticket and explain your problem.

The right way was to grant a cross-account access from you secondary account to the management Organization account, as explained in Accessing and administering the member accounts in your organization - AWS Organizations.