'ej goes between sentences or other verbal clauses.
jIghung 'ej bI'oj I am hungry and you are thirsty;
jIghungchugh 'ej bI'ojchugh, Qe' wISuch If I am hungry and if you are thirsty, we visit a restaurant.
je as a noun conjunction goes at the end of a list of noun phrases.
Soj HIq je food and alcohol
Soj vISoppu'bogh HIq Datlhutlhpu'bogh je food which I have eaten and alcohol which you have drunk
je as an adverbial goes after a verb to mean also, too. There is usually some element of the previous sentence that is being repeated.
jISoppu'. bISoppu' je. I ate. You ate too.
Soj vIje'pu'. vISoppu' je. I bought the food. I ate it, too.
Have you been reading the tips and notes for each unit?
The tips and notes for the very first unit, "Phrases" (aka "Useful phrases") have a section for Joining nouns with and without "and": https://www.duolingo.com/skill/kl/Useful-phrases/tips-and-notes
The tips and notes for the "Basic" (aka "Basic sentences 1") skill have a section for Joining two sentences with "and": https://www.duolingo.com/skill/kl/Sentences-1/tips-and-notes
Together, they should answer your question.
If you haven't found the tips and notes yet -- for most users, they are only available on the website https://www.duolingo.com/ , not in the mobile apps.
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I strongly recommend reading the tips and notes before starting a new lesson unit.
If you have questions even after reading the tips and notes, please do comment on what seems to be missing or contradictory; we would like to improve notes, though occasionally we run into space constraints.