A thumb was a type of finger.
Lepscillians had a massage technique in which the thumbs were pressed against the temples. This made a human go to sleep and feel refreshed when they woke up. (PROSE: Forever Autumn)
The Ralafeans had four thumbs and one finger on each hand and invented the mobile phone before the printing press. (PROSE: Wooden Heart)
After turning into a dinosaur due to the Eleventh Doctor's interference in prehistory, Amy Pond complained that she couldn't lock her phone because she no longer had thumbs. (COMIC: Short-Change)
The "thumbs up" gesture was used to indicate approval. It was also used as an emoji: 👍. (TV: Smile)