File Block: Button and Title Aligning to the Right After Saving

I’m sharing another page with you because I changed the previous one: https://prueba12014.wpcomstaging.com/featured-image/.”

It’s likely due to your theme’s lack of proper styling for these elements. It’s difficult to say since your CSS has been optimized in way that obscures its true source.

You can patch your theme’s lack by adding your own CSS to the Additional CSS section of the customizer or style book. For example:

.wp-block-file {
    flex-direction: column;
}

You can further style the elements as desired, for example:

.wp-block-file a {
    text-align: center;
}

I would like the link and the button to be side by side, like the default design. Is that possible?

I’m unsure what the default design looks like, but I’ll take a guess.

Change flex-direction property from column to row and add the following below it:
flex-wrap: wrap;

Then add:

.wp-block-file a:nth-of-type(1) {
    flex-grow: 2;
}

This makes the caption bigger than the button.

Keep the text-align styling. You can fiddle with flex-grow properties and nth-of-type (1 or 2) to get the relative proportions as you prefer. Your browser’s element inspector tool can help you determine the right values to use. Any changes in the tool will appear immediately on the page. Changes in the tool will not persist, copy the desired rules to Additional CSS for persistence.