If I perform a back up using finder of my iphone 8, will ALL of my phone be backed up?? I am specifically concerned about photos, text messages, contacts, voicemail phone messages, apps....

Backup methods for iPhone, iPad, and iPod touch - Apple Support

Backups from your computer

A computer backup of your device, which is not the same as a sync, includes almost all of your device's data and settings. A backup from a computer doesn't include:

• Content from the iTunes and App Stores, or PDFs downloaded directly to Apple Books
• Content synced from Finder or iTunes, like imported MP3s or CDs, videos, books, and photos
• Data already stored in iCloud, like iCloud Photos, iMessages, and text (SMS) and multimedia (MMS) messages
• Face ID or Touch ID settings
• Apple Pay information and settings
• Apple Mail data
• Activity, Health, and Keychain data (to back up this content, you'll need to use Encrypted Backup in iTunes.)

No.