Links to information on GnuPG – Encryption & Signing
As of Thunderbird 78, Thunderbird no longer uses GnuPG / Enigmail. Thunderbird now has OpenPGP built in. you can read more here.
This infographic (click for larger version) gives a brief over view. It can be found here (click infographic) However that page also has a guide on how to set it all up.
Key signing can also be a part of any GNU / Linux user group or Raspberry Pi jam and this is essential to make the web of trust work, ensure the person whos e-mail you sign is really that person. By checking their ID then you can flag software to trust that person at which ever level of trust you select. From untrusted to a higher level of trust.