Html snippet to all of the posts

Various options:
– Create a child theme, details here:
https://codex.www.ads-software.com/Child_Themes
then customise the post template (name details here):
https://developer.www.ads-software.com/themes/basics/template-hierarchy/
– Use a hook function or filter, details here:
https://adambrown.info/p/wp_hooks/hook
https://adambrown.info/p/wp_hooks/hook/filters
You may need to read your theme files to find the filter/hook to use.

In case you do have some php coding experiences, just add this function to your child theme (or wrap it in a plugin format and use it).

function my_add_after_content($content) {
	if( is_single() && is_main_query() ) {
	$new_content = '<p class="myaftercontent">This is to added after content</p>';
	$content = $content . $new_content;?? ?
	}?? ?
	return $content;
}
add_filter('the_content', 'my_add_after_content');

But honestly, I would use this kind of plugin, Add Widget After Content, that adds sidebar area to the end of post so that we can use text widget (which now offers html and image functionality) in it.

Add Widget After Content
https://www.ads-software.com/plugins/add-widget-after-content/