First of all, thank you for your patience and time.

My plan is a character witch may look a little bit too focused and cheater.

He is a Dragonborn (blue-bronze) sorcerer with the storm sorcery magical origins.

My question is, what if he becomes a cleric with tempest domain?

Will the special effects (like divine domain and sorcerous origin) work with the other class spells and the breath weapon as well?

P.S: Let me ask, how many cantrips can a spellcaster use? I couldn't find it. I searched it in the PHB but it said: "Repeated practise has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over."

Its a doable combination

level 2 character sheet for a level 1 sorcerer/level 1 cleric available here if you want a base to work from: https://drive.google.com/file/d/1qGYz1IPq7cD8kwkvVEcSpaCuYDfJD0wF/view?usp=sharing (although it does need a background added and skills chosen, it also uses the standard stat array of 15, 14, 13, 12, 10 8 to determine ability scores so you could roll them or use whatever method your DM prefers)

Effectively the tempest domain and storm sorcerer abilities work with each other, just bear in mind though that if bounce between the class equally and get to level 20 you end up with a level 10 storm sorcerer level 10 tempest cleric you'll only have level 5 sorcerer spells and level 5 cleric spells but you'll still get access to level 6, 7 8 and 9 spell slots so its worth taking a few spells that can be cast at higher levels.

It might also be worth starting of as a cleric to get armour and straight away just to increase your early game survivability..

From a cantrip point of view, you can cast one cantrip you know per round using a bonus action or action as noted in the spell description but you can them over and over and are effectively a range of "at will" spells that you can access.

Thank you so much for your answer.

I came up the same consequences, that starting az a cleric will be better. We plan our game sort so I don't think about top-level spells.

You can know as many cantrips as your classes give you. If you know 4 sorcerer cantrips and 3 cleric cantrips, you will know 7 cantrips total. They can be used at will by using your action (or rarely, your bonus action).

I'm playing a Storm Sorcerer - Tempest Cleric multiclass right now actually. I'm a Minotaur with a focus in midline combat with my lance. The other night we leveled up to level 11 so I've been playing him for some time now. This multiclass is very powerful as their abilities match and combo perfectly. At 12th level, with a 6 and 6 blend of each, this is the optimal control tank with meta magic like twin spell and extended spell, you can cast cure wounds from a distance, or twin heal yourself or others quickly. This isn't to include stuff like twin spelling chromatic orb with elemental adept, ensuring great damage rolls, or a nice shatter using channel divinity. With hitting 5th level in sorcerer the other night I now have access to 3rd level sorcerer spells giving me lightning bolt. I can now cast this using a 6th level slot and channel divinity to guarantee 33 damage on a save or a full 66 damage on a failure. Lastly, the ability to move and maneuver enemies on the battlefield is amazing. I can pull or push enemies away from weakened allies, knock creatures of the deck of our boat, you name it. Is this build powerful? Yes. Is this build fun? Yes, yes it is.

(also, starting as a cleric doesn't really matter as sorcerer gives you con saves prof which is really good for keeping concentration. Also tempest cleric gives you prof in heavy armor just by delving one level into it so... no cons here!)

It sounds impressive...

I'd second the recommendation  of starting as a sorcerer for the proficiency in Con saves. We're playing a lvl 20 one-shot soon and one of my friends is playing a 19 storm sorcerer/1 tempest cleric. The flight, armor proficiencies, and high-level spells all make a really powerful package.