ReCAPTCHA analyses your general activity on pages up to the point where a human/bot decision needs to be made, in order to figure out whether to show you the "Please select all images..." challenge.

reCAPTCHA: No CAPTCHA reCAPTCHA (v2+):

In 2013, reCAPTCHA began implementing behavioral analysis of the browser's interactions to predict whether the user was a human or a bot. The following year, Google began to deploy a new reCAPTCHA API, featuring the "no CAPTCHA reCAPTCHA" — where users deemed to be of low risk only need to click a single checkbox to verify their identity. A CAPTCHA may still be presented if the system is uncertain of the user's risk; Google also introduced a new type of CAPTCHA challenge designed to be more accessible to mobile users, where the user must select images matching a specific prompt from a grid.[1][20]

In 2017, Google introduced a new "invisible" reCAPTCHA, where verification occurs in the background, and no challenges are displayed at all if the user is deemed to be of low risk.[21][22][23] According to former Google "click fraud czar" Shuman Ghosemajumder, this capability "creates a new sort of challenge that very advanced bots can still get around, but introduces a lot less friction to the legitimate human."[23]

And based on my experience with other sites where the ReCAPTCHA challenge page was sprung on me unexpectedly, but empty, if you block the Google bits that host it you are simply unable to proceed with the process it's protecting.

As for the people who don't have access to a computer, I guess that's what the "119" phone number is meant to be for. If/when they get round to adding the "collect code" functionality to it.