Зачем нужны метки {0}, {1} ну и такие метки

Answer 1

string s1 = "Привет ";
string s2 = "FanTop";

Console.WriteLine("{0}" + "{1}", s1, s2); //Привет FanTop

Где {0} и {1} маркеры подстановки, а s1 и s2 элементы подстановки

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string s1 = "Hello ";
string s2 = "FanTop";

Console.WriteLine("{0}" + "{1}", s1, s2); //Hello FanTop

Where {0} and {1} substitution markers, s1 and s2 permutation elements

Answer 2

They are called placeholders.
They hold the data you want to insert into the string.

It is just a way of doing it.

Strictly speaking you do not need it.

You can use
String concatenation
Placeholders
Or string interpolation

There are several ways to do things right.

https://code.sololearn.com/cZAfo3Y9ElXv/?ref=app