Is tefillah (prayer) a Torah-level or rabbinic obligation?
The obligation to pray is from the Torah ("serve Him with all your heart"). But the three daily times, the Amidah's 18 blessings, and fixed texts were established by the Anshei Knesset HaGedolah.
Question 2
Which Patriarch is associated with Shacharit (morning prayer)?
Avraham established morning prayer, Yitzchak established afternoon (Mincha), Yaakov established evening (Maariv).
Question 3
What does "service of the heart" mean?
The Rambam explains prayer as the "avodah shebalev" — the heart's service, which corresponds inwardly to what sacrifices accomplished outwardly.
Question 4
How many extra prayer services exist beyond the three daily ones?
Musaf is added on Shabbat and Yom Tov (corresponding to the additional offering). Neilah is added only on Yom Kippur.
Question 5
Who fixed the texts of the prayers as we know them?
The Anshei Knesset HaGedolah, led by Ezra, codified prayer texts so that everyone — even those who didn't know how to pray properly — could fulfill the obligation.