Yes, that's one of the fundamental usages of these two characters. Their most basic usages in detail: 是: 1). Judge word. Almost equals to link verb "be". States that something has some properties, or something exists in somewhere. The basic form is "subject+是+noun" or "subject+是+adjective". e.g. "我是男孩"(wǒ shì nán hái, I am boy). "他们是英国人"(tā men shì yīng guó rén, They are British). "中国菜是好吃的"(zhōng guó cài shì hǎo chī de, Chinese dishes are delicious. And in fact, the more natural way to express this is to change "是" into "很" and delete "的" at the rear of an adjective: "中国菜很好吃" zhōng guó cài hén hǎo chī) And when it comes to formal subjects begin with "There be", "是" should be transformed into "有" to illustrate the existence of the object. e.g. "桌子上有一本书"(zhuō zi shàng yǒu yì běn shū, There is a book on the desk). "操场上有一群人"(cāo chǎng shàng yǒu yì qún rén, There are a group of people on the playground). 2). Link word for alternative questions. Basically used in the form "是...还是..."(...or...) . e.g. 你是要吃饭还是面(nǐ shì yào chī fàn hái shì miàn, Do you want to eat rice or noodles). 在: 1). State the existence of the subject in somewhere, without "there be". Basic structure: "subject+在+place". e.g. "我在教室里"(wǒ zài jiào shì li, I am in the classroom). "他在家"(tā zài jiā, He is at home). See? There is always an adverbial of place behind "be". 2).The symbol of progressive tense. Basic structure: "subject+在+verb+object", equals to "subject be doing sth." e.g. "我在吃水果"(wǒ zài chī shuĭ guǒ, I am eating fruits). "他们在踢足球"(tā men zài tī zú qiú, They are playing soccer). That should be the basic usages of "是" and "在" for a beginner's level. As for more advanced usages, I'll talk about them sometime later. I hope you can find that helpful.

it's correct to see it the way you see it. 在 can also works as a proposition, like "at" in English.

they are all correct.but 他门（wrong）,他们（right）.

'zai' (1).prep.means 'at'. For instance,i am (at) home now. means 我在家 (2) =ing. mostly like '(正)在'.For instance, i'm having dinner. means 我（正）在吃晚饭。 'shi' mostly used in a definition.

people use 在when they are trying to express ongoing things. like "ing "in Chinese. 是is present tense. describing facts