To upload 15,000 records in every single .xlsx sheet that contains 100 different xlsx sheets in a SharePoint List where every new sheet will append, you can follow these steps:
Prepare the data in an Excel file with 22 columns and 15,000 rows. Save the file as a .xlsx file. Open SharePoint and navigate to the list where you want to upload the data. Click on "Add new item" and select "Import spreadsheet". In the "Import Spreadsheet" window, enter a name for the new sheet and click "Browse" to select the .xlsx file. Click "Import" to start the import process. Repeat steps 4-6 for each of the 100 sheets that you want to append the data to. Note that SharePoint has a limit of 5,000 items per list view, so you may need to create multiple views to display all the data.
Also, keep in mind that uploading a large amount of data to SharePoint can take some time and may require additional resources to handle the processing. It is recommended to test the upload process with a smaller subset of the data first to ensure that everything is working correctly before uploading the full set of 15,000 records.