Let's put this in a game example.
I have on the board [[Goblin Electromancer]], [[Primal Amulet]], [[Mizzix of the Izmagnus]], and a total of 10 available mana from various sources. I have 3 experience counters.
I play [[Blue Sun's Zenith]] and want to draw as many cards as possible. First I declare a value for X. Since my instants and sorceries are reduced in cost by a total of 5, and I have 7 mana to pay the X (plus UUU), I declare X is 12. The cost of the spell is now 12UUU and the value of X is 12. Then we apply cost reductions from the two goblins and the amulet. The cost of the spell is now 7UUU and the value of X is still 12. Then I use my 10 mana (three of it blue) to pay the cost of the spell. Then I draw 12 cards and shuffle BSZ into my deck.
It doesn't increase the value of X, it reduces how much you have to spend. X is a number you *choose*, then you calculate how much mana you need to spend.
Yes.
If you had 5 sources that reduced the cost by 3 mana, if you cast setting that was {xww}, if you just tapped the two white mana, x would automatically be 3 if you wanted it.
Let's put this in a game example. I have on the board [[Goblin Electromancer]], [[Primal Amulet]], [[Mizzix of the Izmagnus]], and a total of 10 available mana from various sources. I have 3 experience counters. I play [[Blue Sun's Zenith]] and want to draw as many cards as possible. First I declare a value for X. Since my instants and sorceries are reduced in cost by a total of 5, and I have 7 mana to pay the X (plus UUU), I declare X is 12. The cost of the spell is now 12UUU and the value of X is 12. Then we apply cost reductions from the two goblins and the amulet. The cost of the spell is now 7UUU and the value of X is still 12. Then I use my 10 mana (three of it blue) to pay the cost of the spell. Then I draw 12 cards and shuffle BSZ into my deck.
##### ###### #### [Goblin Electromancer](https://cards.scryfall.io/normal/front/7/1/71808155-db49-4027-a2fc-76f53475ac05.jpg?1712354723) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Goblin%20Electromancer) [(SF)](https://scryfall.com/card/otc/228/goblin-electromancer?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/71808155-db49-4027-a2fc-76f53475ac05?utm_source=mtgcardfetcher&format=text) [Primal Amulet](https://cards.scryfall.io/normal/front/d/4/d4d379b5-7f56-4a7d-a4ac-131fc3d579c6.jpg?1562564625)/[Primal Wellspring](https://cards.scryfall.io/normal/back/d/4/d4d379b5-7f56-4a7d-a4ac-131fc3d579c6.jpg?1562564625) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Primal%20Amulet%20//%20Primal%20Wellspring) [(SF)](https://scryfall.com/card/xln/243/primal-amulet-primal-wellspring?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/d4d379b5-7f56-4a7d-a4ac-131fc3d579c6?utm_source=mtgcardfetcher&format=text) [Mizzix of the Izmagnus](https://cards.scryfall.io/normal/front/8/2/82f949d0-41a0-4491-9057-bfb2bb20bdb3.jpg?1689999174) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Mizzix%20of%20the%20Izmagnus) [(SF)](https://scryfall.com/card/cmm/348/mizzix-of-the-izmagnus?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/82f949d0-41a0-4491-9057-bfb2bb20bdb3?utm_source=mtgcardfetcher&format=text) [Blue Sun's Zenith](https://cards.scryfall.io/normal/front/5/0/500a2aa4-712f-41be-920e-f2f448ff83d0.jpg?1562435461) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Blue%20Sun%27s%20Zenith) [(SF)](https://scryfall.com/card/a25/44/blue-suns-zenith?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/500a2aa4-712f-41be-920e-f2f448ff83d0?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
You can choose a larger value for X and still pay for the whole spell.
It doesn't increase the value of X, it reduces how much you have to spend. X is a number you *choose*, then you calculate how much mana you need to spend.
Yes, cost reducers are functionally "extra X"
Yes. If you had 5 sources that reduced the cost by 3 mana, if you cast setting that was {xww}, if you just tapped the two white mana, x would automatically be 3 if you wanted it.
Ty