I CAN create multi-column index with table in design view.
ONLY if you use the Indexes dialog was my point. And that dialog is not at all clear as to how this is done. Using just the table design, you can use Shift or Cntl - click to select a group all together or several fields that are not adjacent but this creates a primary key. This click method is not available to create a unique index.
Yes, you can join tables/queries on calculated fields. When I want to do that, I normally create queries to concatenate the fields and then a final query to join the query with the calculated fields but you can do it all in the same query. The point that others made is that doing this prevents Access from being able to use indexes to speed up the join process.
From tbl1 inner join tbl2 ON tbl1.fld1 & Format(tbl1.fld2, "00000") = tbl2.fld1 & Format(tbl2.fld2, "00000")
This assumes that fld2 is numeric. When you concatenate two numeric fields like this, you need to ensure that both fields are the same length.
The first field seems to be fixed at 4 digits but the second field could be different length. So, if you have 2020 and 10 and 2020 and 5, you want the concatenated values to be 202010 and 202005 respectively.