How are isogeny graphs made and how are they helpful to crypto? I don't understand how the shapes of isogeny graphs are determined. While Alice and Bob do walk on it and don't backtrack, are they actually relevant to crypto?

Also, I was told that supersingular isogeny graphs are expander graphs. Similarly, how is the shape of an expander graph determined
Let an elliptic curve $E(F_p)$ have the Frobenius discriminant $D_\pi$, and $\left(\frac{D_\pi}{l}\right)$ be a Kronecker symbol for some $l$-degree isogeny. If $\left(\frac{D_\pi}{l}\right)=-1$, then there are no
$l$-degree isogenies; if $\left(\frac{D_\pi}{l}\right)=1$, then two
$l$-degree isogenies exist; if $\left(\frac{D_\pi}{l}\right)=0$, then $1$ or $l+1$ $l$-degree isogenies exist.


  Therefore, if $\left(\frac{D_\pi}{l}\right)=1$. then $l$-degree isogenies of elliptic curves form branchless cycles, and changing
  direction in a cycle means switching to dual isogenies.


This is a Theorem2 of "PUBLIC-KEY CRYPTOSYSTEM BASED ON ISOGENIES", by Alexander Rostovtsev and Anton Stolbunov.
With this theorem, you can determine the shape of isogeny graphs.