No one knows.
There is considerable TRADITION about them - where they ministered, how they died, etc. But none of that is recorded in books in the NT canon
Perhaps it's important to remember that the Bible is not intended to just give us information - no matter how important WE may think it is. John recorded his purpose with something like "These things are written that you may believe that Jesus is the Christ and that by believing you have life in His name."
Of course, we also see this with Jesus. We are told almost nothing of the first 30 years or so of His life. Four books tell us SOME things about the last 3 years but it's very, very likely that the VAST majority of His preaching, miracles and ministry is not found in any of the four. We are told what we need to be - to fulfill the purposes of Scripture. But little more.
It's customary to hold that Scripture has two purposes: To show us the Savior and show us how to live - Gospel and Law.
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