Today scholars generally agree that Mark was the first of the Synoptic Gospels written. Most scholars also believe that it was written some time after the First Jewish-Roman War, which ended in 73 CE.

I would argue most scholars are not divided on whether these texts were written by primary witnesses. For instance, the name “Mark” comes from an associate of Paul, who—as legend goes—was told these things about the narrative of Jesus by Paul. However, Paul was not a firsthand witness.

Mark 15:21, which depicts a scene from Jesus’s crucifixion, lends a particularly relevant verse regarding your question: “They compelled a passer-by, who was coming in from the country, to carry his cross; it was Simon of Cyrene, the father of Alexander and Rufus.” Note that the author does not give a last name or a geographical identifier (something common in Mark). This causes the reader to infer that Alexander and Rufus were known to the author’s community. There is a Rufus mentioned by Paul as one of his associates in Rom. 16:13, but the connection is impossible to confirm. Regardless, what’s important is that the author within the text is making it clear that this is a narrative about Jesus, not a firsthand account of historical truth.

It is also worth mention that one cannot discuss the Synoptic Gospels without mentioning the Sayings Source Q. Q—from the German “quelle” meaning “source”—is a written document of Jesus sayings that were used by the authors of Matthew and Luke in addition to the narrative framework of Mark when constructing their gospels. Unfortunately, there is not a physical textual document remaining of Q. Nevertheless, scholars have generally reconstructed what they believe was written in Q.

All this is to say that the majority opinion among scholars is called the “Two-Source Hypothesis” and is as follows: foundational for inspiration in the Gospel of Mark were oral sayings traditions. In addition to oral sayings traditions, Matthew and Luke used Mark and Q as their two main sources. Matthew consolidated almost all of Q’s wisdom sayings in what is now commonly called Jesus’s Sermon on the Mount. Meanwhile, Luke used Q more sporadically and characterized Jesus’s teachings as very relevant for his time rather than ALL TIME (note: the author of Luke is also the author of Acts, hence it being commonly referred to as Luke-Acts). Both Luke and Matthew add to and rework the Markan narrative as well, most notably with the birth and death-resurrection stories.

Generally Mark is thought to be written 70-75 CE, Matthew 80-85 CE, and Luke some time after both. Please let me know if you have any more questions, and if I answered thoroughly enough. I want to note that I am saying “generally” and “the majority” so often because within a scholarly field such as the New Testament canon, one is bound to see religious faith interfere with legitimate scholarship, and as a result one can always find conflicting (though not the scholarly consensus) viewpoints.

Edit: In regards to your “non-canonical” gospels question: Mark was the first narrative attempt at Jesus’s life. The Gospel of Thomas, for instance, is simply a collection of Jesus sayings. The infancy Gospel of James is more a story detailing the miraculous tale of Mary. As strange as it may seem to many of us, the idea of writing a “bios” was not a prerequisite for what we might call “pre-Christian” congregating. Rather, there are signs that point to ritual meeting for meals where it may have been common to celebrate and recite hymns concerning Jesus’s death. Additionally, it seems some of the earliest associations of Jesus people met together to contemplate and add to Jesus wisdom sayings.