How to migrate 6 Exchange 2013 servers with 2 DAGs to 2019

[Vihim428]

I have read through some contents for the titled including below and some others
"https://techcommunity.microsoft.com/t5/exchange-team-blog/best-practices-for-migrating-from-exchange...

I know that from different source

1. It works when exchange 2013 has latest CU23 installed.

2. All DAG member should be operating under same OS

2. An exchange server can only be a member of only one DAG

The questions are:

1. In a DAG that has 3 2013 nodes, should i (or is it possible to) add the 2019 to the DAG,  or

should I remove DAG member until there is only 1 member, move DB, uninstall 2013 exchange, on 2019 recreate DAG? because i can only find some DB movement on the internet but no info about DAG.

2. As there are two DAGs, is it possible to merge it up? 

Many thanks to all

[#2]

The Exchange Server 2019 servers with new mailbox DBs need to be in a new DAG and you migrate the mailboxes from Exchange Server 2013 to the new DBs on Exchange Server 2019.
The Exchange Server Deployment Assistant provides more guidance https://learn.microsoft.com/en-us/exchange/exchange-deployment-assistant?view=exchserver-2019

[#3]

@Dan Snape 

so i should not do or think in a way that i am moving the DB to new DAG but

create new DAG and new DB, then move the mailbox from old DB to new DB?

[#4]

Correct. There is no supported way to move a mailbox DB from one DAG to another. You have to move the mailboxes from the old DB/DAG to a new DB/DAG