I don't understand all the intracacies of tire sizes. I seem to remember that with the 20" on my Bike Friday, there are two incompatible standards. Finding tires (years ago) was not a problem and I'm running a great pair of Schwalbe Marathon Supreme, iirc.

Question #1 - Do 18" tires also have two different standards?
Question #2 - recommendation for 18" tires and sources. I've got room to go wider than 1.5, btw, so whatever I can find that's close.
Question #3 - what does 40-355 (on sidewall) mean?

What I have is a "Birdy" road tire, with sidewall numbers: 18x1.5 and 40-355
I have googled all of the above and have gotten nowhere useful. There seems to be a significant shortage of options, and tires in stock.