I have a friend who practices 8 hours on weekends or holidays. I also heard that japanese practices for 7-8 hours a day (according to movie I watched) . What are they actually practicing? My friend is a serious pianist and often win first places. I believe my friend practiced efficiently. But what did he/she practice? What takes so long?

When my teacher (we both take tuition from the same teacher btw) isn't satisfied about my pieces, she'd say I need to practice with metronome (including slow speed) or just left hand, or just right. But is that it? She said it'll take about 3 hours to completely master such thing (EACH) but my friend practiced many hours in total before competitions. Is that all she practice? Repeatedly?

I hope there are suggestions of what to practice because I don't understand why people practice so long and what they practice. Thank you!