The "Post mosaic" widget of Elementor is added by the Pro.Radio Elementor plugin and is useful to display blog posts in a "mosaic" composition, that requires the featured image as background.
This item requires a full width column (cannot be used in a half or third column). You can only display blog posts, not other custom post types.
How to add a "Post mosaic" widget
Step 1: Edit your page with Elementor
Step 2: search "mosaic" in the widgets
Step 3: Drag on your page the "Post Mosaic" element.
Step 4: category filters - extract the posts within a specified category
Step 5: Posts by ID - set a list of IDS separated by a comma. If used, any other parameter will be ignored. Example:
128,234,934
Step 8: "Order by" - Read more in the paragraph "Ordering Widget Results" within the page "4.2 - Elementor pages".
Step 9: "Offset" - skip a specified amount of results from the first.
Step 9: "Exclude" - you can remove from the output certain items by adding their IDs in this field, comma-separated like this:
128,234,934