Several popular WP plugins that can help create custom fields, including 1) Advanced Custom Fields (ACF), 2) Pods, 3) Meta Box, 4) Toolset Types – Custom Post Types, Fields, and Taxonomy, 5) JetEngine – WordPress Advanced Custom Fields, and 6) Custom Field Suite. Below is one tutorial article that teaches how to create custom fields using ACF: Using the Advanced Custom Fields Plugin to Create Custom Fields
Custom fields can also be created without plugin(s). Below are few tutorial articles that show how to achieve this goal:
How to Create Custom Meta Boxes & Custom Fields in WordPress?