Every, like each, is always used with a singular noun form and therefore with a singular verb form in English because we are counting the things or people that we are talking about separately one by one:

• Every child in the class plays a musical instrument.
  
• Every instrument belonging to the school is tuned regularly.

everybody / everyone / everything

It is true that everybody and everyone refers to everybody and everyone in a group of people and everything refers to everything in a group of things. Even so, they are still perceived individually or as a totality, so the singular verb form is still maintained:

• Is everybody happy with that? Does everyone understand my position?
  
• Everything she owns has been stolen.

all - with singular verb forms

All is more often used with plural verb forms, though sometimes it is used with singular verbs. This happens when we are referring to all as a totality of items under consideration. Here, all is close in meaning to everything:

• Is all well with you?
  
• Don't cry! All that matters is that you are safe.
  
• That is all she has in her suitcase: a blouse, two dresses and a pair of sandals.
  
• All she wants now is to get back home to her parents.
  
• All that happened was that she got on to the wrong train.

Note that in this sense, all is often used together with a relative that-clause, all that matters, all she wants, etc. In your example, Ihsan, as there is no relative clause following all, we would be more likely to say:

• Is everything finished? ~ Yes, everything is finished.

All - with plural or singular verb forms

However, if we wish to specify the things or people under discussion, we can use all or all of with nouns and pronouns and the correspondingly correct singular or plural verb forms. Compare the following:

Is everything finished?
Has all (of) the cake been eaten?
Has all of it been consumed?
Has it all been demolished?
Have all (of) the presents been distributed?
All (of) my friends have come to my party.
All of us are going to Sam's party next week. We're all going.
All of them were singing Happy Birthday.
They were all singing Happy Birthday when the lights went out.

Note from the above examples that all and all of are mainly used with uncountable (e.g. cake) and plural (e.g. presents, friends) nouns. With personal pronouns, two realisations are possible, e.g. all of us or we all.

all - for emphasis

Finally note that all can be used for emphasis with certain adjectives and adverbs, prepositions and conjunctions. Here, all is close in meaning to completely:

• Your feet are all wet. Have your boots got a hole in them?
  
• I got scared, ran off and left her all alone.
  
• All around me I could hear people calling her name.
  
• Tell me all about your kayak trip up the estuary..
  
• I got sunburnt and it was all because I didn't have a wide-brimmed hat.