adjust the observable sequence

[LeoJiang1997]

now i facing a problem is that how to adjust the sequence of observable that are being sent.
for example:

a().subscribe(v => {
b().subscribe(() =>
c());}
)

The current sending order is aaabbbccc, I need to adjust it to abcabcabc

https://stackblitz.com/edit/angular-ivy-1vehgd?file=src/app/app.component.ts