According to your description, you are doing cluster OS rolling upgrade.
2012R2 and 2016 nodes can run in the same Cluster as long as it is at the 2012R2 Cluster functional level. 2016 and 2019 hosts can run in the same cluster as long as it is at the 2016 Cluster functional level. These are the only combinations you can have when there is a mix of OS's. So, we need to go with Server 2012 R2 -> Server 2016 -> Server 2019.
And please refer to the following article learn the detailed upgrade steps:
Thanks for your time!
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