I was checking my toyota celica's coolant and it look empty, until i opened a second cap on the bottle, why?

Sounds to me like an airlock in the system. you will have to check it when its cold by opening the engine coolant cap on the engine, not the filler bottle and top it up if needed also squeeze pipes while you do it

If even possible, it sounds as though your water
pump is not turning the right way

It isn't even moving any water & that is all
you worked on, unless the thermostat somehow
could go in backwards

try to open the air filter and pour a little bit of petrol inside carb, bottle cap size... if its runs, then your carb need to service, the car might not starts for quite sometimes i guess.

You can put it either one or both. If you can see coolant in the bottle with the "engine coolant" cap ( resevoir) then there is no need to open the other. Fill the resevoir to the full mark. If that bottle is empty then you can open the radiator cap ( do not open hot). Do it when the engine is cold because the antifreeze in the rad can be hotter than boiling water and is under pressure. When the engine is cold, turn the cap until it stops, then push down and turn a little more and it will come off. There is a safety tab on it to keep hot coolant out of your face. Fill the radiator slowly until the coolant is about 1/2 to 1" below the neck and reinstall the cap. Make sure you get it on properly, both safety tabs are caught and fully tightened or it may blow off. Then you can top up the resevoir too. Hope this helps.

1. When setting the temperature to MAX, blower level to FULL and outside air Vent CLOSED, no hot air comes out after 2-5 minutes?
If YES, when did you last flushed your radiator and is it full?
2. if more than 5 years ago, flush radiator system, instructions are usually in Owners manual. Use a bottle of Radiator cleaner, run egine hot for about 10-15 minutes, carefully open petcock in bottom of radiator to flush out radiator, WARNING, careful as flushed coolant would be boiling hot. As sson petcock is open and water start to flow out, carefully open radiator cap to enhance flushing flow. Use garden hose to fill radiator while flushing until water coming out from petcock is cold and clear.
Once clear, start engine, turn heter full and blower full blast and keep hose in radiator flowing full, leave petcock open until clear water flows out again. Once clear waters comes out, turn off engine,turn off garden hose and let water drain empty radiator. Once it stops flowing out, close petcock snug and put radiator cap on, pull off radiator coolant catch bottle, clean bottle for any debris and dirt. Drain any water, leave bottle empty and put it back onto its place.
3. Fill radiator with 1 gallon (4L) of concentrated coolant, top up radiator until full, fill catch bottle with concentrated coolant, start engine and keep it overflowing until no bubbles comes out and the radiator is fully hot. Once it is flowing good, close and connect everything. Drive with it and keep catch bottle filled as necessarry.
Good luck. Give as vote and good comment if this advise solve youor problem. Thank you.

If you dont have an external coolant leak anywhere under your hood that has shown up on your driveway it could be the intake manifold or the headgasket leaking. Also check the thermostate to solve the heat issue.

FILL RAD AND BOTTLE, LEAVE CAP OF AND WATCH WHEN ENGINE IT RUNNING , IF LEVEL DROPS WHEN MOTOR REVS PUMP OK, IF NO CLEAR MOVEMENT PUMP FAULTY, IF BLOWS WATER OUT, POSSIBLE HEAD GASKET OR CRACKED BLOCK/HEAD, YOU CAN DO THIS WHEN COLD,
ALSO CHECK THERMOSTAT IS NOT UPSIDE DOWN AND ALL HOSES CONNECTED ON REBUILD ON CORRECTLY

you can add it to the radiator ( if the engine is cold) or look at the top of the radiator for a small hose...follow it to a plastic tank...check the cap on the top if the tank...it should say "engine coolant fill" or something to that effect...remove cap , and fill to the line....2 lines...marked hot or cold..depending on if the engine is at normal operating tempature...DO NOT OPEN THE RADIATOR CAP IF THE ENGINGE HAS BEEN RUNNING.. you can get burned really badly

EmWiz is correct. You have to remove the coolant overflow bottle, and RAISE IT UP to eliminate the trapped air in the system. The heater core is above the height of the radiator, and air tends to get trapped in the core. Raising the overflow bottle is the only way to purge the trapped air. Keep adding coolant to the overflow bottle until the bubbles disappear, and you have good heat at idle.

I hate to tell you this but its possible...

You have a leak. Look around on the driveway and try to determine if you have an external leak as opposed to an internal leak. If you see no coolant on the ground, suspect a blown head gasket. Do not continue to drive this car in this condition. If it does indeed have a blown head gasket, serious engine damage will occur.

Check your exhaust for white smoke. Chances are good you have a blown head gasket and coolant is leaking into the engines combustion chambers and is essentially burning off. Also check your oil, if it looks like caf mocha, this will confirm coolant is leaking into the oil via the blown head gasket