Confusions:

1. It's not just TCP packets flowing through tun0. It could be UDP and ICMP too.
2. UDP is the protocol that encapsulates your OpenVPN traffic

Question answers:

1. The 54.202.18.143 ... eth0 line is your default route. This is where any traffic that does not match a better routing rule will be sent.
2. All packets except the OpenVPN UDP encapsulation traffic will go through tun0
3. Only the OpenVPN UDP encapsulation traffic will go via 54.202.18.143.

Confusions:

1. It's not just TCP packets flowing through tun0. It could be UDP and ICMP too.
2. UDP is the protocol that encapsulates your OpenVPN traffic

Question answers:

1. This is an explicit route to the VPN gateway (the 255.255.255.255 ensure that 54.202.18.143 matches exactly that one IP address: nothing more and nothing less):

    54.202.18.143   10.0.2.2        255.255.255.255 UGH   0      0        0 eth0
   

   I assume that 10.0.2.2 is your LAN router, so this is telling your client that all traffic for the VPN endpoint is to be sent out via eth0 to your router (and onward). This ensures that because your pseudo-default route has been updated to send all traffic via your VPN, the encapsulated packets themselves don't get routed through the VPN too. (Like a snake eating its tail, this is not a good situation to be in.)

   The pseudo-default route is built from these two lines:

    0.0.0.0         10.8.0.5        128.0.0.0       UG    0      0        0 tun0
    128.0.0.0       10.8.0.5        128.0.0.0       UG    0      0        0 tun0
   

   These have a higher priority than your real default route but lower than priority than any other lines in your routing table. Together they match all traffic except that which matches other lines in your routing table.

2. All packets except the OpenVPN UDP encapsulation traffic will go through tun0

3. Only the OpenVPN UDP encapsulation traffic will go via 54.202.18.143.