Questions about zombies spawn

[TSOC_yaya]

I cant upload the pics, so I make a link : https://ppt.cc/fWG1Hx

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The little blue squares is 11*11 blocks, and all 141*141 blocks were covered with forged steel.

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When blood moon started, the zombies spawn not to much while I stand on A area, and sometimes they even try to break walls to got me instead of walk through the S shape road. The way to solve is stand on B or C, and C seemed much better. I've heard that zombies won't spawn on man-made blocks, so my questions are below.                                                                  1. If the zombies would spawn around me, how long is the max distance?                                                                                                                                                                                      2. If I put dirt(I made) on forged steel block, would it count as natural blocks to spawn zombies? And If I delete the forged steel blocks, then put dirt(I made) back, would it count as natural blocks to spawn zombies?

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