SharePoint Framework to read term from TermSet

Difference between spfx web part and extension:
Web parts allow you to create components that can be added by content editors as content on a page. Extensions add functionality to known areas of pages on an entire site or site collection, for example to add things in the header or footer of every page, or add menu options in a list toolbar. Extensions are often added by administrators.

Source: What is difference between SharePoint Framework (SPFx) Extensions and SharePoint Framework (SPFx) Web PART

You could use pnp js get terms:
https://pnp.github.io/pnpjs/v1/sp-taxonomy/docs/term-sets/