For the first question, I’m not sure how the value is being submitted so I’m assuming that $_GET[‘press’] is an array of the selected values, if this assumption is incorrect then you’ll need to alter that part to match the submitted value.
foreach ($years as $key => $value) {
foreach ($value as $year) {
?>
<option value="<?php echo $year; ?>"<?php
if (in_array($year, $_GET['press'])) {
echo ' selected="selected";
}
?>><?php echo $year; ?></option>
<?php
}
}
To show the posts for the selected years you need to create a pre_get_posts filter that alters the WP query. https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts. Check out some of these topics on using pre_get_posts https://support.advancedcustomfields.com/forums/search?bbp_search=pre_get_posts
