AWS confirguration for multiple SFTP.

Looking for assistance with an Ubuntu 20 server hosted on AWS (t3a.xlarge) configured for 6 to 8 SFTP users. Each user accesses their respective directory via FileZilla using individual passwords for uploading and downloading Excel files. The challenge lies in integrating an ERP system for one specific SFTP user who provided a PUB file (public key) for passwordless access. This user needs to connect to the server without a password or PEM file, while the other users will continue using passwords for login access via SFTP. The task involves implementing this specific PUB file for seamless access without affecting the existing password-based access for other users.