Indoors refers to closed places or buildings actually it's usage is mostly for places with doors so that you won't get confused while using it just use it for places with doors and then for inside it carries the meaning of indoors but also you can use it for everything to express the meaning of being in or into something

"indoors" only refers to into or within a building. "inside" can be inside anything. ( as well as a building) "Let's play indoors!" ("Let's play inside!" is also correct) "What does an apple look like from the inside?"