You have some options here to achieve this: (1) Write a PS Scripts that reads your Excel files and upload the data to a SPO list (2) Upload the Excel files in SPO and use Flow to access the content in the files to store them in the SPO list
Quick edit is how I usually do it. Make sure you use Classic mode thou. I've noticed trying to paste in more than 20 records in modern quick edit gets stuck on an indefinite loading box. But classic mode works fine, so click bottom left to go classic, then the list / library tab, and you'll see Quick edit.
This is assuming these spreadsheets are not too complicated data wise ;).
@Freenit It all depends on your data and some answers are already provided as well.
If you are concerned about accuracy then I will either go with
A] Power shell
B] Small Console Application
But if you can take risk with Accuracy then
A] Copy the data from different excels to diff. lists (hopefully some correlation will be their to makes tows unique) and then write a workflow or ms flow which triggers and copies that data from different list to a single list. Why I said the accuracy can be challenging if you have some conflicting data you need to handle carefully.
B] Manually coping with quick edit its painful and leads lot of errors.