Learn Spanish (Latin America): Los pases se validan al entrar. - The passes are validated upon entering.

This sentence uses the reflexive passive (pasiva refleja). In this structure, the thing receiving the action (los pases) is actually the grammatical subject of the sentence. Because los pases is plural, the verb must match it and be plural (validan). Literally, the structure functions as if saying 'the passes validate themselves.'

Could I use the regular passive voice and say Los pases son validados al entrar?

You could, but it would sound formal or bureaucratic. The regular passive voice (using ser + past participle) is less common in spoken Spanish. For general statements where the person doing the action isn't specified, native speakers overwhelmingly prefer the reflexive passive (se validan).

Is it okay to change the word order and say Se validan los pases al entrar?

Yes, absolutely. In fact, putting the se + verb combination before the subject is extremely common in Spanish, especially when the subject isn't a specific person. Both word orders are completely natural, and putting the verb first simply places a slight emphasis on the action itself rather than on the passes.

Why do we use the infinitive entrar after al instead of conjugating it?