In these visualizations, the Replace step is done automatically for you.

You will have to trigger each Apply step by clicking on the value which should be applied to a lambda variable.

This expansion logic uses the convention of always applying or replacing the leftmost thing you can operate on.
See the main page for a short overview of what is going on: what's Lambda Calculus, what are Replace and Apply, what value should be applied to a lambda variable and why, etc.

Expression:

Symbols: