It’s available for android, ios, and free on itch(dot)io if you want it free, but Mindustry. Maybe not as free and creative as Scrap Mechanic (still have absolutely NO clue where to even find it for purchase), but it at least has a mode where you can build your own levels if you get bored of the official levels.

For more creative ventures, you could, and I can’t believe I’m saying this, go with minecraft (or luanti with a bunch of different mods/games from the content store if you’re looking for something free).

Edit: don’t know what I was thinking. Found Scrap Mechanic on Steam almost immediately. Must have been thinking another game or something else.