Put it like this:
The stress is always on the second-to-last vowel.
The vowels in Esperanto are: a, e, i, o, u.
(The consonants are: b, c, ĉ, d, f, g, ĝ, h, ĥ, j, ĵ, k, l, m, n, p, r, s, ŝ, t, ŭ, v, z.
Note that both j and ŭ are consonants! Example: ankaŭ has the stress on the first a - Ankaŭ - not "ankAŭ".)
To answer your question:
Skip "syllables". Count the vowels (a, e, i, o, u) instead.
tiu has the stress on i
tiuj also has the stress on i
I believe that when the -j plural marker is added on, the syllable to which it is attached becomes a diphthong, meaning it is still one syllable with a sort of 'blended together' sound. So if "tiu" is pronounced like "TEE-oo", the word "tiuj" would then be pronounced "TEE-ooee".
A similar effect occurs with the 'a' and 'ŭ' in "aŭto", or just "aŭ". Alone, the 'a' is just an "ah" sound, but when 'ŭ' is added to it, there's still only one syllable, but pronounced together like "ow" (as in "out").
In fact, you can -- you just can't do it in the Duolingo course. The course makes a very strict difference between "this" and "that" which is not always observed in reality.
Occasionally when I say this, my comments will get voted down (presumably by people who have learned in the course that "Ĉi tie" means "here" not "there"), but nobody has yet to give an example of a single appearance of a demonstrative word like this, appearing by itself with no other demonstratives as contrasts, where this distinction matters.
I can point to a spot right in front of me on a table and say "look there, there's an ant on the table!" and nobody will question it.
So, back to the original sentence, if I were to come up to a bunch of cars for sale, I could very easily drop the ĉi. But, if the conversation was more like this:
- Ĉu tiuj aŭtoj estas bonaj?
- Ne, ili estas malbonaj, sed ĉi tiuj aŭtoj estas bonaj.
... in this case, you can't drop the ĉi.
See here for more info on correlatives: https://en.m.wikipedia.org/wiki/Esperanto_vocabulary#Correlatives
I don't understand your reference to "people and things". Is that in the Tips and Notes? It sounds like you're quoting something, but I can't tell what. Here's what you need to know to see how this works:
Point 1: Tio (and related words like io, kio, ĉio, and nenio) will take an -n if they are a direct object, but they never take a -j ending.
Point 2: Tio answers "what" and tiu answers "which" or "who".
Keep in mind also that we sometimes say "what car" when we really mean "which car."
So - which cars are good - those cars are good. It needs to be a form of tiu.
Another way to put all this is that if "this" comes before a noun (expressed or implied) it needs to be a form of tiu.