How to Create Re-Usable and Lookup Drop-down Lightning Web Components?

Salesforce announced Lightning Web Components (LWC) in December 2018, with the comparison of the existing Aura Framework.

What Is Lightning Web Components?
Lightning Web Components is a new programming model for building Lightning components. It leverages the web standards discoveries of the previous five decades, may coexist and interoperate with the initial Aura programming version and also delivers exceptional performance. Lightning Web Components are custom elements built using HTML and modern JavaScript. Lightning Web Components take the Salesforce lightning platform one step closer to using web standards for UI development. It is beneficial for Salesforce lightning developer and The main benefit of using LWS is, it is fast, more secure and lightweight.