How to secure a WordPress website?

Keep the core, themes and plugins updated, and delete the ones you do not use.
Use strong and different passwords, and two factor login for admins.
Give each user the lowest role needed.
Use HTTPS and the latest supported PHP version.
Limit the login attempts, and do not use the username admin.
Turn off the file editor: define( 'DISALLOW_FILE_EDIT', true );.
Set the correct file permissions (755 folders, 644 files, 600 or 640 for wp-config.php).
Take regular backups and test the restore.
Download plugins and themes only from trusted places, and never use nulled ones.
Use a security plugin or a firewall for scans and brute force protection.

You can check if your core files are changed:

Terminal output

$ wp core verify-checksums
Success: WordPress installation verifies against checksums.

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