Learn Finnish: Puolue kampanjoi joko kadulla tai netissä. - The party campaigns either on the street or on the internet.

They translate to "either ... or". Just like in English, joko ... tai is used to pair two parallel options. In this sentence, it links two grammatically equal location phrases: kadulla (on the street) and netissä (on the internet).

In English, we say "on the street" and "on the internet". Why does Finnish use different case endings (-lla vs -ssä) for these?

Finnish local cases depend on how a space is physically or culturally conceptualized. Katu (street) is treated as an open surface, so it takes the adessive ending -lla. Netti (the internet), however, is treated as a metaphorical container or environment you are "inside" of, so it takes the inessive ending -ssä.

What kind of verb is kampanjoi, and why doesn't the last vowel double for the third-person "it" form?

The base form is kampanjoida (to campaign), which is a Type 2 verb. To form the third-person singular for puolue (the party), you simply drop the -da to get the stem kampanjoi. Because this stem already ends in a diphthong (two vowels), you do not add an extra vowel to the end.

Both kadulla and netissä look slightly different from their base words katu and netti. What is happening here?