This is the inside of a homemade unit.
Pros. Cheap to make
Cons. Low water volume
Poor heat transfer due to water moving from the hottest part of the coolant to the coolest and back again.
This is a plate heat exchanger.
Pros. great heat exchange
High flow
Cons. more expensive due to need for extra fittings and takes some creativity to mount.
The one below is how most exchangers work.
Pros good waterflow
good heat exchange
Very economical
Cons. not as easy to make