varii

No -ari- in there. Esperanto has -ar-, the collective suffix that turns arbo into arbaro, a forest, and vorto into vortaro, a dictionary. It does not have -ari-, and varii is not var- plus anything: varo means a commodity or an item of merchandise, and goods for sale have nothing to do with variation. Vari- is one borrowed Latin root, and the double i in the infinitive (vari + i) is what happens when a root already ends in i.

That double vowel is worth a second. It is not a typo and it does not contract: varii, varias, variis, varios. Other roots in the same shape behave identically - studi is fine, but kopii gives kopias and linio gives linia. Esperanto never drops a letter to make a word prettier.

Intransitive, and this is the trap. Varii means to be various, and nothing does it to anything else. Prices varias; the weather varias. You cannot varii a menu.

To vary something on purpose, you need -ig-: variigi la menuon is to make the menu various, which is exactly what varying it means. This is the igi / iĝi contrast doing the job it always does, and varii is a clean example because English hides the difference behind one verb form.

Against its neighbours.

• Ŝanĝiĝi - to change. One thing becomes another over time.
• Varii - to vary. Several instances differ from one another, or one thing differs from occasion to occasion. It implies a range, not a transition.
• Diversa - various, the adjective for the same idea.

Derived forms. Vario and variado are variation, varia is variable as an adjective, variaĵo is a variant, and variablo is the mathematical variable. The opposite is konstanta.