Okeydokey something near and dear to my heart.
Both DC/DC chargers and isolators do the same thing at the elemental level. They serve the function of separating house batteries and starting batteries. But how they do it is a bit different.
A conventional isolator (solenoid/mechanical switching) keep all batteries (house and starting) connected to the alternator when the vehicle is running or voltage is above a certain point. As stated they work best when all batteries are the same size/type/age/rating. Basically you have one big battery bank when the vehicle is running, and when the vehicle is off, the connection is severed, allowing you to drain the batteries you have selected as house batteries and keep the starting battery(s) full. Advantages are they are reasonably cheap and simple, and the good ones like the Blue Sea isolators let you switch to "start on house batteries" in case your starting battery dies. The big disadvantage in modern systems is that a conventional isolator offers no step-up voltage, the best you can ever get is whatever the alternator puts out which will leave lithium batteries undercharged.
A DC/DC charger does things a little differently, the house battery is still charged from the alternator, but the output from the alternator is never ganged to both banks of batteries without going through the DC/DC charger. The advantage of this is that you can run batteries with different charging schemes (voltage, float, amperage, etc) between your house battery and your starting battery to take advantages of both types of tech, IE you can run lithium house batteries and AGM starting batteries and everyone is happy. DC/DC chargers are also all solid state, so no moving parts. They either work or they don't. The disadvantage to DC/DC chargers is they do not let you easily connect ALL batteries in the system to start off your house batteries, you would have to move wires around to do that. But thats neither here nor there because you can't start off a lithium battery in most cases. The other advantages of some DC/DC chargers is they also have built in MPPT solar charging and can float your starting batteries off the solar once your house batteries are topped up. That feature is real nice for storage if you have solar access. Because DC/DC chargers have the ability to step up voltage to the house batteries to charge them, you can get a full charge of batteries that may require more charging voltage than your alternator can put out.
A Smart Alternator is an alternator that will decrease output to reduce fuel consumption once batteries reach a certain charge, this is troublesome because the alternator can be functioning as intended and the output voltage is low enough that it won't trigger the "vehicle is on, start charging" mode of a DC/DC charger. To get around that, a switched signal wire needs to be added to the DC/DC charger telling basically, "Yes I know the voltage is pretty low, but I assure you the vehicle is running and it is OK to charge now"
Lithium has several advantages and disadvantages over AGM.
Cost is getting to be close, but lithium will still cost more.
Lithium is 100% safe to be mounted indoors, no vents or any hanky-panky.
Charge and discharge rates on lithium batteries is a lot higher, meaning you can pump 50 amps into them safely and reliably and charge a 100Ah battery from flat to full in 2 hours. Can't do that with an AGM. You can also discharge them at a higher rate which is nice if you want to run an inverter or other sustained high load device.
You can't start a vehicle from a lithium battery, at least not a high draw vehicle like a diesel.
Lithium batteries do not perform well in the cold. Specs vary, but they can not be charged below a certain temperature and can not output below a certain temperature. AGM batteries will lose amperage and capacity in extremely cold weather but will still perform.
