I have an Excel document with 750000 rows and the requirement is to create a custom list in SPO with this data. I am on SharePoint Online.
You can export the Excel data to SharePoint Online using combination of Microsoft Access along with Excel. In this approach, you need to create a list with all the necessary schema and use MS Access to map the data from Excel to SharePoint List.
You can refer to the Approach 3 in below article:
You can try the script given in below articles
You can use the option 'Export to SharePoint' from Excel.
For more details you can visit this article
Although you CAN do this. You MAY want to look at other options for the data. If you choose to load it into SharePoint as a list you will have to manage views of the data so that you can avoid the inevitable list view threshold.
Ask "Why does it NEED to be in SharePoint?" if the answer is "So we can view it" or "So we can report on it" then you may want to look at loading it into Azure SQL and visualize the data in a web part.
Ultimately, you will be dissatisfied with that many items in SharePoint and how cumbersome it will become to manage.