Tutorials on how to make modular maps? + Buildings

Sorry this is more than one question! So I really want to expand my skillset and learn how to do environments and try my hand at buildings to sell in the marketplace but I haven't the slightest idea on how to create assets that are meant to be repeated or rotated and fit. I've tried to find tutorials and I only found a few on youtube but I'm struggling to understand on how to put it all together and trying to watch videos on how to make repeating patterns. Were there any specific tutorials or resources that you guys used when you first started out making maps and environments? It just seems so overwhelming but I want to be able to have more variety. I understand you want to make things their own file so you can reuse assets and objects in future maps, but the terrain and architecture is a whole other animal to me. Also can you make a mini pack in the marketplace like a small map that would include some assets and some creatures? I couldn't really find anything except for entire mini adventures. 

You make repeating patterns by offsetting the pattern by 50% to the horizontal axis, pasting it in again and putting it 50% the other direction, then using the clone tool to fix the seams. Then do the same vertically. Then do this once more to test to make sure there are no seams visible in the repeat. Offset and check how the seam looks. If it needs to be fixed, then fix it. That's basically it. You can make a set of base template alpha shapes for roads if you wanted to do a seamless road kit, for example. Or you can make a consistent shaped rectangle with consistent exit points and occasional 'end pieces' that way it's always the same layout to journey throughout your sets even if the texture doesn't mesh. But if you aren't into it, why not stick with what you are enjoying? No sense suffering if you get more joy from drawing monster tokens.