Learn Swedish: Cykelbanan är belyst. - The bike lane is illuminated.

You are right that Group 2 verbs usually form the en-word past participle with -d (like stängd). However, the verb belysa (to illuminate) belongs to Group 2b. Because its stem (belys-) ends in the voiceless consonant s, the ending changes to -t to make it easier to pronounce. As a result, the past participle is belyst for both en-words and ett-words.

Is "är belyst" a passive verb construction or an adjective here?

It acts as an adjective describing the current state of the bike lane. Using är (is) plus a past participle focuses on the result of an action. If you wanted to focus on the action itself happening right now (e.g., "The bike lane is being illuminated"), you would use the passive s-form of the verb instead: Cykelbanan belyses.

The word belyst looks related to lysa (to shine). How does the prefix be- change the meaning?

Lysa means "to shine" and cannot take a direct object (e.g., the sun shines). Adding the prefix be- often turns a verb into a transitive one, meaning it now takes a direct object. So belysa means "to shine light on something" or "to illuminate".

What is the base word of cykelbanan, and how is it different from en cykelväg?