Google finds 10 million documents that mention "o defesa" in the context of football. I believe the difference is that "o defesa" refers to a defence player rather than the general concept of the noun "defense" which would be "a defesa"
This section of the course is about sport, with heavy emphasis on football. And I lost a heart :-) because I used the most relevant for this context.
Please correct me if I'm wrong.
Yes, "o defesa" refers to a defense player, but it's not a common term, I don't see it being used frequently, at least in Brazil. People prefer to refer to a player by their specific position ("o zagueiro", "o lateral") rather than "o defesa".
If ‘o defesa’ refers to a defence player, rather than the way a team goes about defending or the group of players in a team that are primarily on defence, the translation would be ‘the defender’, I think.
Yes. But the direct translation for that would be "o defensor". Like I said, I rarely, if ever, heard "o defesa" being used.