Getting false for is_home() on Posts page

Question

I've made a child theme of Twenty Twenty theme and want to display the posts/ category archive pages in flexbox grid.

However, none of the conditional tags I'm hoping to check outside the loop (is_home, is_archive, is_category) are returning true on posts page or category archive pages:

* @since Twenty Twenty 1.0
*/
get_header();
?>

<?php if ( is_archive() ): ?>
    <h1>ARCHIVE</h1>
<?php endif; ?>
<?php if ( is_category() ): ?>
    <h1>CATEGORY</h1>
<?php endif; ?>
<?php if ( is_home() ): ?>
    <h1>HOME</h1>
<?php endif; ?>

<main id="site-content maybe-flex-directive-class-here">

    <?php

    if ( have_posts() ) {

        while ( have_posts() ) {
            the_post();

I expect either I'm missing something obvious or going about it the wrong way.

Answer 1

I was going to (and actually did) delete this post, but after sleeping on it, think it might end up being of some use.

What I was missing, above, is [Template Hierarchy][1]. I was looking at the singular.php template file, which will only ever be loaded when is_archive(), is_home(), etc, aren't true.

What got me is that the HTML <main> element exists on both the index.php and singular.php documents in this theme.

So I was able to update the child theme's copy of index.php:

<main id="site-content" class="flex-container">

With CSS:

.flex-container {
  display: flex;
  flex-direction: column;
}
/* Responsive layout - make multi-column on larger screens */
@media (min-width: 800px) {
  .flex-container {
    flex-direction: row;
  }
}

Thanks for the solution in the comment, @bosco.

  [1]: https://developer.wordpress.org/themes/basics/template-hierarchy/