I would like to know how I can create upload only permission level so users are only able to upload documents.
Try to create a custom permission level with at least
add and view items as the following;
To create a permission level
- Open the site collection site (the root site not a subsite)
- Go to site setting.
- below Users and Permission > Click on Site Permission.
- From the above ribbon, > click on permission level to be able to create a new permission level.
For more details check
Create a new permission level named Uploaders and check the permissions Open, Add Items, View Items and View Pages. Then grant the permission level to the users and groups who should be able to upload documents but not edit and delete them.
You can manage permission levels from Site Settings > Site Permissions > Permission Levels
Be careful, as this doesnt stop them from being able to click on New/ document, or folder, ss etc, so not quite an "upload only" permissions, sadly.