Translation:They turn on computers and learn English.
they turn on the the computer. Implies many people turning on 1 computer. Is it to mean this way?
No. Unless there is a determiner (this/that) and there is no plural indicator (cái máy tính đó - that computer), this sentence should indicate a number of people turn on a number of computers.
"They" could also be "they/them/their" (unless there's a genderless designation?)
Or "they turn on the computers and learn english" also wrong. In this case leaving out the classifier in english makes this sentence American and i am Australian. Duo needs to do some English lessons.
I'm Australian too but I don't think "They turn on computers and learn English" is natural even in American English. It needs some kind of determiner before "computers". Either "the" or "their" work great.