$1.$ If the Internet is an IP$-$based network, why do we even need PPP$?$

$2.$ If PPTP and L$2$TP do not actually secure data themselves, then why do they exist?

$3.$ If PPTP and L$2$TP basically do the same thing, why choose L$2$TP over PPTP$?$

$4.$ If a connection is using IP$,$ PPP$,$ and L$2$TP$,$ where does IPSec come into play?