MTG Salvation

Hello communities.

Recently, I encounter Garna, the Bloodflame and after few search, found that you can built a deck around it. I found combo Deck who use card like Thermopod, Ashnod's altar and Sneak attack, but I dont understand how that work. I f you can enlightened me about all the combo I can do with the card, I would be thankful.

I don't play this commander, but what I can work out from these particular cards is that you can stack Garna triggers so that you can infinitely use the Sneak Attack and get the mana to pay for it with either Thermopod or Phyrexian Altar.
You'll want a creature that has some sort of 'enter the battlefield' or 'leaves play' in order to gain advantages as well.

For example lets pick Zulaport Cutthroat and Phyrexian Altar.

So you have Sneak Attack in play.
Sneak Attack in Zulaport Cutthroat.
Sacrifice to Phyrexian Altar for R.
Cast Garna, The Bloodflame. -or- if in hand use the Sneak Attack to put into play.

Now this part is important, while Garna, The Bloodflame trigger is on the stack, you sacrifice Garna, The Bloodflame to Phyrexian Altar. This will mean that Garna will return to your hand when his ability resolves.
You get both Zulaport Cutthroat and Garna back to your hand.
You of course have a R for each creature you sacrifice.
So this means that you can keep putting creatures into play for R and instantly get the mana back from sacrificing to either Phyrexian Altar or Thermopod.

This is infinite. So it's just a case of having another creature that actually does something with infinite.
For example Solemn Simulacrum will allow you to get all your basic lands, and draw as many cards as you want.
Priest of Gix will give you infinite black mana, while Priest of Urabrask will give you infinite red mana.

Note that Ashnod's Altar is colorless mana, so I don't see how this pays for Sneak Attack unless you have something else. Only Phyrexian Altar I think will work this way.
Priest of Urabrask or Gemstone Array in conjunction with Ashnod's Altar would pay for the R for Sneak Attack.
Actually any sacrifice outlet is going to work with Priest of Urabrask and Sneak Attack, so if you use something like Altar of Dementia or Blasting Station you should win the game.

Hope that makes sense, I can explain it in more detail if required, but as I say the important part is making sure Garna, The Bloodflame also goes back to your hand, by sacrificing him before his trigger resolves on the stack, so that he will also return to your hand along with anything else that's gone to your graveyard that turn.