I’ll take up this case. Matthew didn’t copy Mark. All ancient sources agree Matthew came before Mark basically without exception, and when the texts are read parallel it actually makes more sense to say Mark is copying Matthew and Luke.
In the text, it is a very common trend that Matthew will include a detail, Luke another, and then Mark will include both details - even when they’re redundant. Compare:
Mark 1:32 - “That evening, after sunset, people brought to Him all who were sick and demon-possessed.”
Matthew 8:16 - “When evening came, many who were demon-possessed were brought to Jesus...”
Luke 4:40 - “At sunset, all who were ill with various diseases were brought to Jesus…”
For Mark to be written first, then the argument would have to be made that Matthew and Luke both read Mark and coincidentally (or intentionally) chose exact opposite details from one another to retain. One would also have to argue that redundancies like, “That evening, after sunset…” were the product of the first author writing on his own, whereas Matthean priority can explain them as a part of Mark’s synthesizing process.
Modern scholars often assume Markan priority (though the consensus is cracking) for two reasons. First, it’s shorter, and their assumption is that shorter means earlier because that is how Germanic fairytales developed. But that neglects the possibility of a work being intentionally shorter unto literary ends. Mark is a fast-paced book. Just go through and see how many times the word “Immediately” appears. So being shorter fits its faster pace.
The other reason they state it is earlier is because it emphasizes Jesus’s humanly characteristics (“lower” Christology). They assume divinity was a later development, and that thus the more humanly the depiction the earlier the text. But this actually inverts what we know about historical debates within the First Century. The early First Century Church primarily had to deal with Rabbinic Judaism, which rejected Christ as God. Meanwhile, in the later First Century we know the rise of Docetism (the belief that Jesus only appeared to be human) challenged His humanity. It makes more sense that earlier texts would emphasize His divinity polemically against the Jews and later texts His humanity against the Docetists.