Office 365 - New customer - Questions

I don't think I am understanding all your questions but check out this resource, which goes through the details of migrating from Google email to Office 365 - https://support.office.com/en-US/article/Migrate-G-Suite-mailboxes-to-Office-365-665dc56c-581c-4e35-8028-6bc1e8497016. One of the first steps is to https://support.office.com/en-US/article/Migrate-G-Suite-mailboxes-to-Office-365-665dc56c-581c-4e35-8028-6bc1e8497016, assuming you wish to use a custom domain, which is purchased separately.  There is a https://support.office.com/en-gb/article/Domains-FAQ-1272bad0-4bd4-4796-8005-67d6fb3afc5a as well.

Exchange Online is the email service that comes with most Office 365 for business plans: all of the Enterprise plans (E1, E3, E5) as well as  Office 365 Business Premium and Office 365 Business Essentials for example, at no extra cost.