When you delete files from a SharePoint Online site, they go into the site Recycle Bin (a.k.a. the first-stage Recycle Bin). If needed, the user that deleted an item can restore it from the Recycle bin. When items are deleted from the site Recycle Bin, they go into the site collection Recycle Bin (a.k.a. the second-stage Recycle Bin).

Only the SharePoint site collection administrator/owner can view and restore deleted items from the second-stage Recycle Bin. If an item is deleted from the site collection Recycle Bin, or it exceeds the retention time, it is permanently deleted.

Files deleted from SharePoint Online sites are retained for 93 days from the time the file is deleted from their original location. Deleted files will stay in the first-stage Recycle Bin the entire 93 days, unless someone deletes them or empties the first-stage Recycle Bin. If the first-stage Recycle Bin is emptied, the items go into the second-stage Recycle Bin, where they stay for the remainder of the 93 days unless:

• The second-stage Recycle Bin exceeds its quota and starts purging the oldest items*
• Items are deleted by the site collection administrator from the second-stage Recycle Bin

*The second-stage Recycle Bin storage counts against the overall site collection storage quota. The amount of space allocated to the second-stage Recycle Bin is 200% of the site collection quota. These values are not configurable.

Since there are multiple Recycle Bins within SharePoint. It's best to look in the Recycle Bin of the user that deleted the item first for deleted or missing content.  To learn more about this see Article 101281, "Where should I look for missing SharePoint content?"