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.

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.