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For future reference if you want to find verb forms you can use the dictionary of the Real Academia. So if you look up ser which is the infinitive and then press the button marked conjugar you will get the full conjugation. Here it is https://dle.rae.es/ser?m=form#conjugaciongZoQ72j
You will see that it gives two second person familiar singular forms tú / vos and alongside them eres / sos the corresponding verb forms. The use of vos is called, not surprisingly voseo and we have a tag for it voseo which will give you more information. As for the coverage of it this question and answers discuss which countries in Latin America use vos.