Thank you Rick Claus
For the Set-ADDomainMode command and Set-ADForestMode, is it really raising the Domain functional level to Windows2016Domain domain and the forest to Windows2016Forest despite we are using Windows 2019 OS? There's no Windows2019Forest or Windows2019Domain?
Also I have read somewhere to not put the Infrastructure master role on the same DC as the global catalog server. If the Infrastructure master runs on a global catalog server, it stops updating object information because it does not contain any references to objects that it does not hold. This is because a global catalog server holds a partial replica of every object in the forest. But usually all DCs (best practice at least 2 of them) are also GC. Should or could the infrastructure master role be transferred to a server that is not a DC?