Understanding Bootstrap 5 variables import order overrides

I do not understand the following situation. I have this order of SCSS variables imports in my app.scss

@import '~/node_modules/bootstrap/scss/functions';
@import '~/node_modules/bootstrap/scss/variables';
@import '~/node_modules/bootstrap/scss/mixins.scss';

@import 'variables'; // overrides

@import '~/node_modules/bootstrap/scss/bootstrap';

As per the Bootstrap docs, this should be the correct order to import to appropriately override Bootstrap variables. In my variables I have the following CSS:

$font-size-base: 0.8rem;
$font-weight-medium: 500;
$font-weight-semi-bold: 600;

$h1-font-size: $font-size-base * 2.6;
$h4-font-size: $font-size-base * 1.2;
$h5-font-size: $font-size-base * 1.1;

But there is something that confuses me. As you can see the $font-size-base: 0.8rem; is an override, but for some reason, the default variables of Bootstrap do not pick this up. When I wanna have the applied new font-size-base for the default variables of Bootstrap, I need to specifically import them in here also, like so:

$h3-font-size: $font-size-base * 1.75;

Can anyone tell me why that is? I do not wanna import $h3-font-size as the multiplier isn't changing to the default, I just want Bootstrap to pick up the new $font-size-base in their defaults.

Solution

Problem: Bootstrap isn't picking up your SCSS overrides, such $font-size-base: 0.8rem;, therefore you have to manually redefine variables like $h3-font-size.
Cause: This occurs because your overrides are imported after Bootstrap’s default variables, so the default values are already set.
Solution: You must insert your overrides before importing Bootstrap's variables and mixins, per Bootstrap's documentation.

Correct Import Order in app.scss:

// Import Bootstrap's functions first
@import '~/node_modules/bootstrap/scss/functions';

// Override Bootstrap variables here (in your custom 'variables' file)
@import 'variables';

/*
The following variable overrides are as mentioned by you, defined in the 'variables' file:


$font-size-base: 0.8rem;
$font-weight-medium: 500;
$font-weight-semi-bold: 600;

$h1-font-size: $font-size-base * 2.6;
$h4-font-size: $font-size-base * 1.2;
$h5-font-size: $font-size-base * 1.1;
*/

// Import Bootstrap’s variables and mixins
@import '~/node_modules/bootstrap/scss/variables';
@import '~/node_modules/bootstrap/scss/mixins';

// Finally, import Bootstrap’s main styles
@import '~/node_modules/bootstrap/scss/bootstrap';

Why This Works: Since Bootstrap uses the !default flag for its variables, custom overrides (such as $font-size-base) written before importing Bootstrap's variables.scss will take precedence over the defaults.

UPDATE

Based on our discussion in the comments with the questioner, and considering the feedback "Makes sense now, if wanna edit your answer to reflect your comment I am happy to accept it." the updated version of the answer is:

If you need to both override default Bootstrap variables and use Bootstrap’s variables in your custom file, it’s not possible to do so in a single file. Instead, you should separate your custom variables into two files: one for overrides and another for using Bootstrap variables. Import the overrides file first, then Bootstrap’s variables, and finally the file that uses Bootstrap variables after the Bootstrap imports.