How does a worshiper recite behind the Imam in audible prayers, and what should he recite if he joins while the Imam is reciting a Surah after Al-Fatiha, and should he make up what he missed with Al-Fatiha and what is easy from the Quran, or with Al-Fatiha only?

It is not permissible for the congregant to recite behind the Imam if he can hear his recitation. Rather, he must listen attentively, and the Imam's recitation counts as his own recitation. This applies to everything except Al-Fatiha.

As for Al-Fatiha, the preponderant opinion is that it is obligatory for every worshipper to recite it, even if he is a congregant, in both silent and audible prayers. This is based on the Prophet's (peace be upon him) saying: "There is no prayer for one who does not recite the Opening of the Book (Al-Fatiha)," and his (peace be upon him) saying: "Do not do anything except (recite) the Opening of the Book, for there is no prayer for one who does not recite it." This is also based on Allah's (Glorified be He) saying: "And when the Qur'an is recited, then listen to it and pay attention."

Al-Fatiha should be recited during the Imam's pauses, if any, or during his recitation of the Surah. Reciting it during the Imam's pause is preferred. If you join the prayer while the Imam is reciting a Surah, recite Al-Fatiha only, then listen to the Imam's recitation. If you miss rak'ahs, the majority of scholars hold that the recitation (the Surah after Al-Fatiha) should be made up.