How do I get this look? I have a few ideas in mind and looked for something online to help. Then I saw people were using royal icing but this looks like buttercream to me. I plan on practicing before the cake is due but it’d be great if anyone had any suggestions. I plan on frosting my cake and letting it cool for a few hours, add on one layer of mountain scenery, then cool and repeat 1-2 more times. TIA

Your plan would be how I would go about it as well.  Frost first mountain, chill until buttercream is set, frost second mountain, chill until buttercream is set, repeat until done.  I would test your frosting to see if you really need that amount of chill time between layers and if you don't that would help to cut down just how long it will take you.  You could probably even try sticking it in the freezer to help quicken the chilling/setting process.

That is gorgeous and I've never seen it done in buttercream before, only fondant.  This is far prettier!  I agree with you plan.  That is the way I would go about it.