Synergies are highly benefitial card interactions wich are either limited by mana or damage output. In some decks synergies are built in in such a way as to sometimes assemble an infinite combo. In these cases the infinite combos usually include more than 4 cards but since all cards involved are part of the deck's other plans it is not uncommon to assemble.
1) [Sample] Esper Familiars:
This deck operated through cost reduction and casting spells to slow the opponent down to the point of making it impossible for them to win and then closing the game with combat damage or through concessions.
The infinite combos here involve having 2xSunscape Familiar (blue spells cost less to cast), Ghostly Flicker, Mnemonic Wall, one basic island and an outlet such as God-Pharaoh's Faithful (infinite life) or Sage's Row Denizen (infinite mill).
You target the Mnemonic wall or archaeomancer and the island with the flicker which get the flicker back to hand and brings in a blue source untapped which lets you cast the cost-reduced flicker again.
Another infinite combo involves 2xKaroo Lands (Azorius Chancery, Dimir Aqueduct), one Sunscape Familiar, 2xMnemonic Wall, one Snap and an outlet. If another familiar is added then the loop also creates infinite mana of the non-blue colour in the lands. Every time you Snap the Faithful you can cast it again and the Snap with the mana from a Chancery and you also untap another double land. With those 2 mana you cast the Flicker targetting the two walls to get back both the Flicker and Snap. This loop gets you infinite life and if you add another outlet it can mill. You can also target opponent's creatures with the snap until they are cleared, neting a white mana for each creature bounced.
2) [Sample] TortEX:
This deck operates by constantly recurring creatures from the graveyard to grind opponents until you can start dedicating resources to winning. This deck usually features no infinite combos but notable synergies include the following.
Tortured Existence + 2xGolgari Brownscale You gain 2 life for each spent. You can discard Brownscale to get back another Brownscale and gain 2 life.
Tortured Existence + Grave Scrabbler You discard the Scrabbler to TortEx to get a creature to hand and you pay the madness cost to get another creature to hand and the Scrabbler in play. This is the main way to rebuild after a board wipe.
Stinkweed Imp/Golgari Brownscale in the graveyard replace your draws to ensure you always have a creature in hand to use TortEx.
The main grind engine is Crypt Rats, it allows you to have a board wipe in play which can be activated at opponent's endstep or combat phase to ensure that no creature sticks in play. Spore Frog if activated during an opponent's turn prevents combat damage and allows you a bit of freedom before you eventually clear the board with the rats. Rats also deal damage to both players so if you can gain life with Brownscale you can use it to clear the board and win the game.
Tilling Treefolk + Raven's Crime in the graveyard allows you to play two Duress-like spells for each Treefolk cast. With the previously mentioned engines, one can keep the opponent's hand empty to ensure that Frogs and Rats survive until they can be activated during the opponent's turn.
3)Sample Petal Festival
Make lands that tap for a lot of mana, splice Psychic Puppetry onto other arcane spells along the way to generate mana and finally get to a 8 mana+ land to recurr Petals of Insight splicing psychic pupettry along the way to net infinite mana. Then use the mana on Stream of Thought to mill your opponent out.
4)Sample fishelbrand-paupergeddon-oct2019
From the tournament description: "A combo deck that attempts to chain effects that generate a temporary burst of mana and fill its graveyard up. It uses these resources to cast creatures with Delve. Then it plays cards which key off their large mana costs to produce more mana and draw a lot of cards, ending the sequence with either a spell based finish or an assault with the huge Delve monsters."
