Check whether the messages are stored in your Google account (Messages app settings → Backup/RCS settings → enable backup)
Restore messages to the same phone after signing into the same Google account (Messages app → restore/enable backup)
Restore messages to a new phone using Google backup (install Messages app → sign in to the same Google account → restore from backup during setup)
Use Samsung Messages backup if you have a Samsung device (Samsung Messages app or Smart Switch → restore messages from backup)
Export a conversation from the Messages app (open conversation → menu/options → Select messages or Export/Backup → save to device or share)
Use Google Messages web backup options if available (Messages app on phone → Messages for web settings → ensure device is linked)
Pull SMS from the device using a backup file (Android backup extraction tools from a computer; then extract SMS/records depending on backup type)
Use a computer backup you previously made (e.g., phone backup in Google One or device backup) and restore SMS from that backup
If messages were deleted, check Google Drive/Google One backups for the backup date before deletion
If messages were deleted and no backup exists, use forensic data recovery tools on a computer (root access or specialized recovery may be required; results vary)