I was wondering this, too and this helps a lot. I see that you are learning many languages and I have a question; Do most languages interpret their equivalent of "They drink." with drinking alcohol? I know Tamil and English and I know when you are saying someone is drinking alcohol, colloquially, you would say what literally translates to "They are drinking." Also, please excuse any grammar mistakes as I know they are there.
That's just the way it is. Someone learning English could ask why we say "we are", "they are" and "you are".
"Trinken" (the verb "to drink") is a regular verb. All regular verbs get the same endings in present tense. This was introduced in the tips and notes at the beginning of Lesson 1. https://www.duolingo.com/skill/de/Basics-1
If you memorize the endings, then you're golden for all regular verbs.
See above. The short answer is, it's incorrect and that's just how it is. It's just one of those things you have to memorize. It's always going to be "wir trinken", Sie trinken (plural form of Sie-they) er/sie(singular- she)/es trinkt, du trinkst. Once you memorize them, its not bad.
It seems you are not familiar with the concept of verb conjugation (using a different suffix/verb form for I, you, he/she/it, we, they) – is that your question? If so, please check verb tables in online dictionaries (such as https://dict.leo.org).
This is the conjugation for the verb "drink":
to drink (infinitive) = trinken;
I drink = ich trinke;
you drink (one familiar person) = du trinkst;
he/she/it drinks = er/sie/es trinkt;
we drink = wir trinken;
you drink (more than one familiar persons) = ihr trinkt;
they drink = sie trinken;
you drink (on or more respected/non-familiar person(s)) = Sie trinken (mind the capital S, as opposed to sie = she/they).