I live outside of the US and I study at a branch of American university in my country. The university has a platform to connect students and employers. Through this platform several employers text me inviting to apply for a job in their company. However, they require US job authorization, which I do not possess at the moment. To get a visa to work and live in the US, I may need a job offer from the companies in the US, so can I count on those messages to help me to get a visa?
can I count on those messages to help me to get a visa?
No. These messages are not job offers; they are merely soliciting you to apply for a job (and, from your description, only if you are already authorized to work in the US). As you note,
To get a visa to work and live in the US, I may need a job offer from the companies in the US.
"Job offer" means that you've applied and the company has decided to respond favorably to your application and seek to employ you. But it seems that none of these companies are willing to do this. If they were, they would not require work authorization; they would instead be willing to sponsor you for authorization.
Note that such companies will normally still ask whether you have authorization because sponsoring someone for a work visa is fairly costly, so they will take that cost into account in deciding whether to hire you.