How can I get the machine name of a taxonomy term knowing its tid?
The table taxonomy_term_data lists the terms, and taxonomy_vocabulary lists the vocabularies.

In the table taxonomy_term_data, the term string is found in the name field. The field vid identifies the vocabulary assocaiated with the term. And the field tid is the numeric taxonomy id.

So you can look up the relationship between the term name, tid and vid directly from the this table.

Then, to determine the vocabulary name string associated with the term, you need to left join the taxonomy_vocabulary.name field, where taxonomy_vocabulary.vid = taxonomy_term_data.vid.