You always have a transistor working in the amplification region. On the one hand, they have deep negative feedback. On the other hand, they alternate with the upper and lower transistors. At any time, only one transistor is working, and the other transistor is in the off region, but for the whole line, it works in the linear region. So only two people work under suitable working conditions. But one of them has been working, because it is a complementary circuit. For this reason, one of them works in the positive signal area and the other works in the negative signal area.