Learn Danish: Det spændende ved skridtet er forandringen. - The exciting thing about the step is the change.

This is a grammar pattern called abstract substantivation. By using det followed by the definite form of the adjective, you turn the adjective into a noun concept. You don't need to add a word like ting (thing); the structure det spændende inherently means "the exciting thing" or "that which is exciting".

Does the adjective "spændende" get a special ending here because of "det"?

Normally, adjectives get an -e ending after det to make them definite. However, spændende is a present participle (equivalent to English words ending in "-ing"). These always end in -ende in Danish and never change their form, no matter what grammar structure they are in.

Why is the preposition "ved" used here instead of "om" for "about"?

While om is the most common translation for "about" (like when you talk or read about something), the specific grammatical pattern det + adjective + ved is the fixed, standard way to say "the [adjective] thing about [something]" in Danish.

Does "skridtet" mean a physical footstep or a figurative step here?

The root word et skridt means a physical footstep, but it is heavily used in a figurative sense just like in English (such as taking a big step in a relationship or a project). Because the sentence focuses on forandringen (the change), it is almost certainly talking about a figurative step.