Honestly there’s not really an explanation...it just happens bc plot LOL
basically, zhen hao and fu got married in an area that was being investigated for some sort of suspicious cases going around over there which was a slight hinting as to why shen and is daughter switched places. the sneezing is just the way that fu can can switch with shen and zhen hao and fu touching is a way for shen to switch with fu. the "dream" that shen had when he was 19 was memories of his 19 year old self meeting zhen hao in the future. like- if zhen hao had gone back in time and met 19 shen than our present time shen would've remembered him, but instead it was 19 year old shen who switched bodies with his future self which is why he only had a vague memory of zhen hao because them having the rebooted car race had not happened in 19 year old shen's timeline yet.
Okay so I just binge-read this in the last hour, can someone please explain the plot to me. I don't understand the sneezing, going back in time, and all that stuff.