The phrases 'This is it' and 'That is it/That's it' are used in their literal sense to identify a particular object. For example, if you have a box of keys and you are looking for the right key to open a particular door, when you finally find the right key, you would say 'That's it!' or 'This is it!'. 'This' indicates an object close to you, and 'that' indicates an object further away. So if you find the key yourself, you'd hold it up and say 'This is it!' , and if your friend finds the key, you'd point to it and sat 'That's it!'. 'That's it!' also has a idiomatic use meaning or 'It's finished'. When you have finally finished a long job, you might say 'That's it! It's over'. Or when you lose patience, you can say 'That's it! I've had enough.I'm leaving'. Sometimes, 'That's it' in this figurative sense can have the same meaning as 'That's all'. Both can signify the end of something. Note that you can't use 'This is it' in this figurative sense.

Don't make us confusion!

It's hard because these phrases can have various meanings. It depends on the context of the sentence. Example: I see something I'm looking for. Maybe a video game. It's on the shelf in a store. I walk up to the cashier. I point at the game and say, "that's it." Meaning that's the one I want. But it's some distance away so I'm pointing and saying "that's it". Maybe the game is right in front of me. I put my hand on it and say, "This is it." It's closest to me. In my example, "that" is basically referring to the more distant of two things near to the speaker (the other, if specified, being identified by “this”). That's all means to encompass everything. Example: You bring the groceries in the house. Someone asks is there anything left in the car and you say, "No, that's all. Example: Do you have any other questions? No, that's all. You could easily say, "That’s all of it" or "that's all of them" Again, it's another way of saying everything or in its entirety. Hope this helps.

1) That's all. (That is all of <something>) There is nothing else. Usually a reply to a questions like "Is there anything else?" "Are there more <something>?" Sometimes it is used to soften a question when someone is suspicious. Person 1: I would like a hamburger and fries. Person 2: OK. Anything else? Person 3: No, that's all [that I want]. Girl: Why do you keep asking about my sister? Boy: I want to make sure she is safe, that's all. Girl: Oh, I thought you were asking because you think she is prettier! 2) That's it! a) It is said when frustrated. It means you have no patience left and are angry enough to do something. Person 1: I hate it when you talk to me like that. Person 2: That's it! I don't want to marry you! b) It is said when referring to a specific object. Person 1: Show me the car that you want. Person 2: Hmm...I don't see it...oh! That's it! The intonation between a) and b) are very different. If you need a recording of the two, I can help out. 3) This is it. a) Sometimes used to point to close by objects. Person 1: Which card is yours? Person 2: Hmm...this is it. b) Used to focus attention on a moment, usually before some important action. Person 1: Two minutes until we perform on live TV for all of the nation to see. Person 2: OK guys - this is it. Get ready! --- I might have missed a few uses, but I think this should cover it. Sorry if the examples are boring or melodramatic. :) Good luck!