Get Comma Seperated Taxonomy Linked Terms and Last Child Separated By “&” Instead Of Comma

What you’re asking is a generic PHP/programming question, and this: “It links all terms to the last term link only“, is because your code clearly is attaching the link (i.e. the last value assigned to $term_link) to the entire list ($entry_terms), i.e. echo '<a href="' . esc_url( $term_link ) . '">' . $entry_terms . '</a>'.

So what you should have done is, attach the link to the term name on this line: $entry_terms .= $term->name . $end;, like so:

1. Replace this part:

   $entry_terms .= $term->name . $end;
   $term_link = get_term_link( $term );
   

   with this:

   $term_link = get_term_link( $term );
   $entry_terms .= '<a href="' . esc_url( $term_link ) . '">' . $term->name . '</a>' . $end;
   

And then, after that:

1. Replace the $entry_terms = rtrim( $entry_terms, ', ' ); with echo rtrim( $entry_terms, ', ' );.

2. Remove the echo '<a href="' . esc_url( $term_link ) . '">' . $entry_terms . '</a>';.

Additional Notes

1. You should define $entry_terms before the foreach line, i.e. add $entry_terms="";.

2. The ampersand sign is being displayed to the user, hence you need to escape the character, i.e. use ' & ' instead on line 4 in your original code.