Explanation
A direct cause for the "engineers" group having a lot more members than you have added manually is that it is a default group for Jira Software product access. This means that any new user who joins your site and has access to Jira Software will automatically be added to the "engineers" group. You can check and change the default group for Jira Software by going to Products > Jira Software > Manage product > Product roles.
References = [Default groups and permissions] and [Manage role and product permissions]

Explanation
An Atlassian Cloud organization can have one or more sites, each containing one instance of each product1. A site has its own URL and administration settings2. Since your organization has two instances of Jira Software and two instances of Confluence, it must have at least two sites to host them. For example, one site could have the URL acme.atlassian.net and contain Jira Software and Confluence, while another site could have the URL acme-dev.atlassian.net and contain another instance of Jira Software and Confluence.
The other options are not necessarily true for your organization. Having multiple sites does not imply that you have verified your domain, which is a process that allows you to manage the Atlassian accounts of your users and apply security policies to them3. You may or may not have verified your domain, depending on whether you have access to your domain's DNS or HTTPS settings. Verifying your domain is not required to use Atlassian cloud products.
A landing product is the product that your users see first when they log in to your site4. You can only set the landing product to Jira or Confluence. Therefore, you cannot have exactly four landing products, as you can only have one landing product per site. You may have two landing products if you set one for each site, or you may have none if you do not set any.
A Premium subscription plan is a plan that offers advanced features, unlimited storage, 99.9% uptime SLA, and 24/7 Premium support for Jira Software, Confluence, Jira Service Management, and Jira Work Management5. You may or may not have a Premium subscription plan for your products, depending on your needs and budget. Having multiple sites or products does not automatically mean that you have a Premium subscription plan.
A sandbox is an isolated environment where you can test and experiment before making changes to production6. A sandbox is available for Jira Software, Jira Service Management, and Confluence with Premium and Enterprise plans6. You may or may not have sandboxes for your products, depending on whether you have a Premium or Enterpriseplan and whether you have created a sandbox from your organization. Having multiple sites or products does not automatically mean that you have sandboxes.
References :
Understand Atlassian sites and organizations
What is an Atlassian organization?
Verify a domain to manage accounts
Set your site's landing product
Atlassian Cloud Premium
Manage product sandboxes

Explanation
Removing a user from an Atlassian Cloud Organization does not affect the subscription plan for the products.
The subscription plan remains the same until you change it manually or it expires. The billing preview shows the estimated cost for the next billing cycle based on the current number of users and the subscription plan. If you remove a user, the billing preview will reflect the reduced cost, but the subscription plan will not change12 References = Manage product access, Manage your bill for Standard and Premium plans

Explanation
With a Standard subscription plan, you can add multiple products to your existing site, such as Jira Service Management1. You can also add Jira Software to a new site, if you need to create a separate instance for a different team or project2. However, you cannot create or use sandboxes, which are only available for Premium and Enterprise plans3. You also cannot change release tracks, which are only available for Enterprise plans4.
References: Manage your bill for Standard and Premium plans, Add a new product to your organization, How many instances of a product does my organization need?, Manage product sandboxes, Release tracks for cloud products

Explanation
When you delete a group from your organization, the group name and its members are removed from your organization's directory. However, the group name and its members are not deleted from the products that the group had access to. Therefore, if you recreate a group with the same name, the group will automatically regain access to the same products and inherit the same product roles and permissions that it had before deletion1. This means that the group membership is automatically restored as a result of recreating the group with the same name.
The other options are not automatically restored as a result of recreating the group with the same name. Access to a User product role, access to a Product Admin role, global permissions, and access to shared filters are all product-specific settings that depend on how the group is configured in each product. They are not affected by deleting or recreating the group from the organization's directory1.
References :
Delete a group