How do you use the $spacer variables in Bootstrap 4.5?

Here's the code for $spacers in Bootstrap's _variables.scss file:

$spacer: 1rem !default;
$spacers: () !default;
// stylelint-disable-next-line scss/dollar-variable-default
$spacers: map-merge(
  (
    0: 0,
    1: ($spacer * .25),
    2: ($spacer * .5),
    3: $spacer,
    4: ($spacer * 1.5),
    5: ($spacer * 3)
  ),
  $spacers
);

Cool, that all looks useful. But how do I actually use these in scss? I've tried $spacer-4, $spacer(4), $spacers(4)... All throw errors :(