Updated: Jan 17, 2020
Lookup relation is a standard field, but if you want to add it to lightning component, coding is required.
This blog provide the necessary code to develop a reusable Custom Lookup component in LWC.
There are 2 Lightning web component.
1. lwcLookup - Contain actual code to fetch records and display as lookup.
2. lookupContainer - Shows how to call lwcLookup and it's properties.
Please visit below link @github.com for code.