.ds xx textmakes text the value of string register xx.
.ds xx " textIf the text starts with a double quote, the double quote is stripped, and the remainder of the line (including leading spaces) becomes the value of the string. (So, to define a string that starts with a double quote, use two.)
\*xinterpolates the value of string register x, and
\*(xxof string register xx (two characters).
.as xx textappends text to whatever value string register xx already has. The double quote hack applies here as well.
The double quotes don't have to be literal. To reliably append the contents of register s1 to register s2, use
.as s1 "\*(s2