On premise exchange 2016 migrate to office 365 progress question

Read about and configure Hybrid: https://docs.microsoft.com/en-us/exchange/exchange-deployment-assistant

If will offer you the best experience, both for you as the admin and the end users. Specifically to moving mailboxes, you can move them one at a time or in batches, details are here: https://docs.microsoft.com/en-us/exchange/hybrid-deployment/move-mailboxes

the information is very useful, may i know that should i use exchange hybrid to migrate mailbox to O365 before or after changing the MX record pointing to O365 ?

It doesnt depend on the MX record, you can have it pointing either on-premises or to O365. Assuming all your mailboxes are currently on-premises, you can leave it as is and only change it after the migration.

understand, and i have one more question, on premise client computer, i was trying to access portal.office.com and login with my domain e-mail address, and i didnt get any credential popup for entering password and logged in to portal automatically, does it means my ADFS is working ? and i see that the user name is showing keith@domain,onmicrosoft.com , does it means i still not set my domain as default domain on O365 portal ?

Those arent questions that I can answer without having a proper look in your environment. There are multiple forms of SSO that you can use with O365, AD FS offers one of them. As long as you can access the portal OK, you can assume it's all properly configured, but generally speaking there are multiple factors to consider apart from the domain.

There are several migartion methods to migrate your on-premises exchange 2016 to Office 365.Cutover migration is one of the simplest method available. Other migration methods are, Staged, Hybrid and IMP

Steps to Move on-premises Mailboxes to Office 365

1. Create Security Group
2. In the 2nd step, create Cutover Migration Batch.
3. Assign Office 365 License
4. After the migration process is complete, you can point to MX and Autodiscover Record to Office 365.
5. In the final step, stop and delete the migration batch from the Office 365

Since there are some issues involved in this manual method, many users also opt for this automated solution to move exchange 2016 to Office 365 i.e, Exchange to Office 365 Migration tool it can easily migrate multiple mailboxes from exchange to Office 365 in quick interval of time. It supports all versions

You should read this article before :

https://docs.microsoft.com/en-us/exchange/exchange-online

Use Exchange Deployment Assistant tool, Just fill your scenario and it will give you the guidance you need.

https://technet.microsoft.com/en-us/office/dn756393.aspx

If you want to migrate mailboxes in batched then you should use Staged Migration method.

I have migrated 10 mailboxes to Office365 from on premise exchange, but still haven't changed the domain to abc.com as default yet, and I see the migrated mailbox show keith.li@xxxxxx.onmicrosoft.com , and I tried to login to O365 portal with keith.li@xxxxxx.onmicrosoft.com and try to login with on premise active directory user password, but it doesn't work, any idea ?