Learn Hardcore Turkish: Yorgun olunca hep esniyorum.

Hep: very common, informal/neutral; here it means always . It can also mean all in other contexts (e.g., hepimiz = all of us).
Her zaman: literally every time/always , slightly more neutral/formal.
Daima: formal/literary always .
Sürekli: continually/constantly (implies ongoing repetition without much break, not just “always”).

It’s an adverbial time suffix meaning when/once/whenever. It attaches to verbs: ol- (to be/become) + -unca → olunca. The vowel changes by harmony:

After a back unrounded/front vowels: -ınca/-ince
After a back rounded/front rounded vowels: -unca/-ünce Because the last vowel in ol- is o (back, rounded), you get olunca.

Why use olunca instead of yorgunken or yorgun olduğumda?

Yorgun olunca highlights the moment of becoming tired and what happens then (a trigger/condition feel). It’s very common and colloquial.
Yorgunken means while (I am) tired, focusing on the ongoing state rather than the onset. Also perfectly fine: Yorgunken hep esniyorum.
Yorgun olduğumda is more formal/explicit and often feels a bit more specific: when I am tired. All three are correct; the nuance is onset (olunca) vs state (yorgunken) vs neutral/explicit timing (olduğumda).

Can I say Yorulunca hep esniyorum instead?

Yes. Yorulunca (from yorulmak, to get tired) directly means when I get tired and is very natural. It’s close in meaning to yorgun olunca; some speakers even prefer yorulunca because it directly encodes the “getting tired” event.