>>1
You would have gotten more out of SICP if you had paid attention while reading it.
https://mitpress.mit.edu/sites/default/files/sicp/full-text/book/book-Z-H-12.html#%_idx_1062
The way this happens is that the let expression is interpreted as an alternate syntax for
((lambda (<var1> ...<varn>)
<body>)
<exp1>
<expn>)No new mechanism is required in the interpreter in order to provide local variables. A let expression is simply syntactic sugar for the underlying lambda application.