Where exactly does ShortPixel store the data on the database?

Starting with version 5, ShortPixel Image Optimizer (SPIO) stores necessary information on your WordPress database in the following way:

• The tables shortpixel_meta and shortpixel_folders are created by ShortPixel and they contain your settings for the Custom Media Folders: Images outside Media Library
• The image optimization data is now stored in the custom table shortpixel_postmeta
• For now, the old optimization data (stored in the postmeta table) is left untouched, but you have the possibility to remove it by going to Settings -> ShortPIxel -> Tools -> Remove Legacy Data. We do not recommend doing this operation unless you are 100% sure all the data is fully migrated to the new format.
• The table shortpixel_queue temporarily stores the items that are added to the processing queue, until their processing is finished.
• The plugin settings are in the table options . All the ShortPixel entries are prefixed with the string wp-short-pixel .
• It creates an option called shortqwp_spio, the queue system our plugin uses to properly process all images in the correct order.

Here's where ShortPixel Image Optimizer (SPIO) used to store all the necessary information on your WordPress database, prior to version 5:

• The tables shortpixel_meta and shortpixel_folders are created by ShortPixel and they contain your settings for the Additional Media Folders: Images outside Media Library
• The image optimization data is in the table postmeta
• The plugin settings are in the table options . All the ShortPixel entries are prefixed with the string wp-short-pixel .

ShortPixel Adaptive Images stores its settings in the table options , just like SPIO, in a single option called short_pixel_ai_options .

Regenerate Thumbnails Advanced uses the shortpixel_queue table to temporarily store images that are in the process of thumbnail regeneration. It also stores the shortqwp_rta option inside the options table.