I visited Japan about two years ago and was kinda shocked at the amount of foreigners (Indian, Bangladeshi, Vietnamese) working everywhere. I gathered from talking to locals that many of them work illegally.
As someone who lived on a military base in Japan when I was a kid, the country does feel like it's changing in places. But other places that are more out of the way still feel like "home", almost unchanged from that time a couple decades ago. (sometimes literally: I visited the area around that base last year and found billboards that were exactly the same as a couple decades ago)
It's true that once we went into the countryside there were a lot fewer foreign workers, but that is also how it is in western countries.
I have no idea about stats of "illegal immigrants" in Japan, because that could mean working outside of your permitted work, having no or an expired residence status, etc.