I think it really depends on where you are. I visited Japan early last year and encountered a decent amount of foreigners when I went to the Snow Festival or when I was in Tokyo. But when I was visiting friends in Fukuoka I don't really remember seeing foreigners, and someone in an elevator seemed kind of shocked to see my wife and me. If you don't want to see foreigners, you need to stay away from Tokyo and visit the many, many other interesting places in Japan.

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)

Hmm, the first city we visited was Fukuoka and basically all 7-11s and the other mini-market chains were staffed by foreign workers. Lots of (Asian) foreigners around the central train station as well. We saw the same in other cities as well - Kobe, Kyoto, Osaka.

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.

The ones working illegally will absolutely not be so publicly visible. They'll be doing construction, factory work, trucking, night work, etc.

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.