Check out example codes for "sql update from two different database". It will help you in understanding the concepts better.

Code Example 1

update table_name1
set column_name = T2.column_name
from database_name.dbo.table_name2 as T2
  where table_name1.ID = T2.ID

Code Example 2

update table1
set col2 = T2.col2
from DataBaseName.dbo.table1 as T2
  where table1.ID = T2.ID and
        table1.col2 is null

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