In “She said she wanted X”, “she said” and “she wanted” are two different clauses — note that they each have an subject, repeated even though it is the same person. We can optionally use “that” to introduce the subordinate clause. In “she wanted to get a new car”, there is only one clause, which includes the infinitive phrase “to get X”. This infinitive phrase is not a clause and does not have a conjugated verb. You already know this, so keep it in mind as we look at your other sentence: In “I saw Tom get into his car and drive away”, there is only one clause (“I saw Tom X”). “Get into his car” and “drive away” are bare-infinitive phrases that complete the meaning of the one and only clause. The work the same as the infinitive phrase in your other sentence (which you understood already).

"Get" cant be conjugated in the past tense because there is the verb "see" before it which is already in the past (grammar rule of two verbs in the same context) And "drive" followed the tense of "get" due to conjunction