Learn Finnish: Tämä laite on yhtä käytännöllinen ja näppärä kuin puhelin. - This device is as practical and handy as a phone.

No, you don't. In the phrase yhtä käytännöllinen ja näppärä (as practical and handy), the word yhtä naturally applies to both adjectives connected by ja. You only need to place yhtä before the first adjective, just like you only need one "as" in English.

Why is puhelin in the basic nominative case here instead of the partitive case?

In equative comparisons using yhtä ... kuin, the thing you are comparing to is put in the nominative case when it is a countable, individual noun like puhelin (a phone). If you were comparing uncountable mass nouns, like water or time, you would use the partitive case instead.

What is the exact difference between käytännöllinen and näppärä?

Käytännöllinen specifically means "practical" or "functional" in everyday use. Näppärä is a bit more colorful and translates to "handy", "nifty", or "clever". It is very commonly used in Finnish to describe gadgets, tools, or solutions that are smart and easy to use.

I recognize the root of käytännöllinen. How is this long word formed?

It is built from the common verb käyttää (to use). This becomes the noun käytäntö (practice or convention). By adding the common adjective-forming suffix -llinen, it turns into käytännöllinen (practical).