Hello, I have a question about when to use these. Because while I learned that "sedang" is used when a verb has "-ing", sometimes the website corrects me and puts the word with the "me-kan" mutation

for example, "They were planning a wedding" ---------> "Mereka merencanakan pernikahan"

reading the topic on the forums that talks about the "me" mutation, I could not find an example similar to this. Any help?