It does, in fact, essentially have an "e" to it. The umlaut (the two dots above the letter) act to essentially add an "e" to the end of the letter it is above. In fact, you may use cheater-German in most schools by typing an umlaut in full. Mädchen becomes Maedchen. A lot of swiss-German uses this exclusively.
Which sound (if any) is used to join two words into a compound word is basically something that you have to learn.
Sometimes, not even all Germans agree on this point -- and sometimes, there are multiple possibilities with different meanings (Gast + Haus can be a Gasthaus "pub, tavern, restaurant" or a Gästehaus "guesthouse (for sleeping)", for example).
Why Orange + Saft becomes Orangensaft but Kirsche + Saft is not *Kirschensaft but rather Kirschsaft, I don't know: it's just something to learn. Kirschensaft sounds like a reasonable word to me: it could be a word, but it just isn't.
Apfel does not end in -e the way Orange does.
I can think of "Garage" (garage in the sense of the place where you park your car), "Rage" (agitation / annoyance), Blamage (disgrace), ... Basically the words that are originally French. Germans kept the soft g, but, unlike the French, they clearly pronounce the last letter e in those words.
For an umlaut on a mac, hold "alt/option" and type "u". Then type the letter you want the umlaut over. There is a whole keyboard full of fun characters that you access with the "alt/option" key. Some, like the umlaut, are diacritics or other marks that superimpose over the next thing you type. For example, the accent aigu is alt/option + e. Some are just less-used symbols. alt/option + s is the ß.
Note: You cannot count compound 'words' as English words, since if you would German (and other European languages) would get an infinite amount of words. This is because you're (as far as I know) allowed to make every logical combination.
Fun fact: Many natives of these language struggle with making compound words as we are confused by English. We start splitting these words because we are not sure if we are allowed to make a compound words of it. This is part of a phenomenon called 'English disease'. Now this won't happen with Orangensaft ( => Orange Saft) that often, but I thought it would be fun to mention.
They're all conjugations, sort of like "adornments" of a verb. They have similarities. For instance "en" in Wir, "st" in du, "e" in Ich, etc. Some verbs (i believe there are 6) have some other rules but most of them follow the same rules. Also some words that end with "s" will have a "t" instead of "st" in the du form since there's already an "s" as a suffix. As to why it's essen for Wir, isst for sie, I guess it's just the nature of the language and you have to memorize it.
why is there an n between orange and saft?
It's a Fugenlaut ( https://de.wikipedia.org/wiki/Fugenlaut ) or linking sound. A bit like the -o- in many Greek or Latin words in English such as "psychology" from psychē + -o- + logos.
When you add two nouns together in German, sometimes they are just joined as-is, but sometimes an -e- or -n- or -s- comes in between the two.
Sometimes, whether or not a linking sound is added can even make a difference, e.g. Gasthaus versus Gästehaus, but usually only one possibility exists.
So you just have to learn the compound words -- that it's Orangensaft and Pflaumensaft with -n- but Apfelsaft and Kirschsaft without a linking sound.