On some themes, the captions are still placed under the pictures by default.
I am not a coder, but you might try to add this code to custom CSS:
.wp-caption .wp-caption-text {
background: transparent;
padding: 2px 0;
color: #616149;
text-align: center;
position: relative;
}
If it does not work, then remove it and try what you can find here:
wordpress.org/support/topic/move-gallery-captions-back-to-below-images/
wordpress.org/support/topic/image-caption-poorly-placed-on-page-gutenberg-annoyance/