On Lisp Ch 17, page 224: https://sep.turbifycdn.com/ty/cdn/paulgraham/onlisp.pdf

CLTL2 Section 22.1.1: https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node188.html#... - Goes into a lot of detail on how the reader works.

The macros most people mean when they talk about Lisp macros are discussed in CLTL2 Ch 8. They are expanded during evaluation or compilation.

CLTL2 CH 8: https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node97.html

Read macros are able to produce (or not produce) lisp objects by taking control of the stream processing for some period of time.

The compiler/interpreter gets as input Lisp data. Let's use READ, DESCRIBE and EVAL to see that working:

    CL-USER 17 > (defun example (&aux input result)
                   (terpri)                    ; newline
                   (write-string "Input> ")
                   (finish-output)
                   (terpri)
                   (setf input (read))         ; reading from input stream
                   (terpri)
                   (describe input)            ; what did we read?
                   (terpri)
                   (write-string "Output>")
                   (setf result (eval input))  ; evaluation
                   (terpri)
                   (write-string "Result>")
                   (print result)              ; printing the result
                   (values))                   ; return nothing
    EXAMPLE

    CL-USER 18 > (example)

    Input> 
    (progn
      (print (+ 1 3))
      (print 'done))

    (PROGN (PRINT (+ 1 3)) (PRINT (QUOTE DONE))) is a LIST
    0      PROGN
    1      (PRINT (+ 1 3))
    2      (PRINT (QUOTE DONE))
    Output>
    4 
    DONE 
    Result>
    DONE

EVAL is then taking code as a list and evaluates it to a value, as side effects there might be printed output of the program.