One of challenges you might face, when you are working in cloud is migration!
when you need to move your users and data from one location (tenant) to another location.
Office 365 is not an exception for this and it does not provide you a fully online and free solution for this matter.

Microsoft offloads all migration to cloud or cloud to cloud to its professional service Fast Track.
But if you have a small organizaiton and have a limited budget to spend for professional services, you can do the migration by yourself.

Steps to follow :

We will asume that we have a tenant with the name of ABC.com and there is another domain name xyz.com in the same tenant.
So we need xyz to be migrated to it’s own tenant (or a different availble tenant).

1-Create a new .onMicrosoft.com tenant ,create all users which will be migrated and assign licenses to the new users
2-Add the new .onMicrosoft.com UPN to outlook for each user’s outlook, who is part of the domain xyz.com
3-Export Emails from the current mailbox(xyz.com) and import it into .onMicrosoft.com using below steps:
Export:

a. At the top of your Outlook ribbon, choose File.
b. Choose Open & Export > Import/Export.
c. Choose Export to a file.
d. Select the name of the email account to export, Make sure that the Include subfolders check box is selected. This ensures that everything in the account will be exported: Calendar, Contacts, and Inbox. Choose Next.
e. Click Browse to select where to save the Outlook Data File (.pst). Enter a file name, and choose OK to continue
Import:

a. At the top of your Outlook ribbon, choose File.
b. Choose Open & Export > Import/Export.
c. Select Import from another program or file, and then click Next
d. Select Outlook Data File (.pst) and click Next.
e. Browse to the .pst file you want to import. Under Options, choose how you want to deal with emails and contacts, then choose Next
f. If you’re importing the contents of your .pst file into your Microsoft 365 mailbox, choose that mailbox(.onMicrosoft.com) here.

4-Remove the domain from the existing tenant :
a. In the admin center, go to the Settings > Domains page.
b. On the Domains page, select the domain that you want to remove.
c. In the right pane, select Remove.
d. Follow any additional prompts, and then select Close
tip:Microsoft will provide you the reason if the removal failed.for a domain to be removed,you need to make sure there is no user or group availble under that domain.Sometimes you need to do this step manually (for the entity which microsft is not able to remove it automatically).

5-Add the domain in the new Tenant:
a. Go to the admin center
b. Go to the Settings > Domains page.
c. Select Add domain.
d. Enter the name of the domain you want to add, then select Next.
e-Choose how you want to verify that you own the domain.
tip:you will need to verify this with the DNS which is holding your domain
-f you will be asked for SPF but as this is static , you can click next and microsoft will be able to identify the records in your DNS.
-g- click finish
6-in outlook remove the old profile and users will be able to connect with abc.com via the new profile.

Thats all!, you are done with migration and all emails are migrated to new tenant.
Please note that in case if you are using oneDrive, you need to sync all your data to local before removing usernames from abc.com tenant. and after the migration is over,you can login with new username and password in onedrive and then drag the local folders into the onedrive and it will automatically sync to the cloud.