What size of compression fitting do I need?

It is important to understand the difference between

Metric thread tube compression fitting size chart - Knowledge

165C-12 Dixon Valve Brass Compression Fitting - Union Elbow - 3/4

Navien NPE-2 Series Condensing Tankless Water Heaters, 50% OFF

A Complete Guide to Compression Fittings

Basics of Compression Fittings

Brass Compression Fittings and Adaptors

Compression Fittings - TC Ltd

Dishwasher Supply Line - Installation

How to Determine Pipe Thread Type and Pipe Size – Fresh Water Systems