"I am eating an egg now."
Translation:Saya makan telur saat ini.
I think "saat ini" is showing and emphasizing that you are doing something "through this moment" (probably since while ago until a few minutes or hours more). While "sekarang" is "now" (either "right now" or "these days" etc.). So they wanted to show the connectivity with english present continuous tense, but actually, "sekarang" is also usable in this sentence in Indonesian.
lagi = again or more. Saya lagi makan telur = I eat more eggs/I eat eggs again. Waktu = time (long(er) period), saat = time (short(er) period, a moment, but it can also be used for when. However ketika = when and better to use to translate when. Kalau saya punya waktu, saya pergi ke pasar. -> When I have time, I go to the marker. Saat ini saya makan roti. Now, I am eating bread. Waktu saya kecil, saya tidak suka roti/Ketika saya kecil, saya tidak sukaroti/Saat saya kecil, saya tidak suka roit. All mean the same; When I was little, I did not like bread. What to use depend on the person, there are subtle differences with no other words (as far as I know).