You can spend 6 months out of the year (and 90 consecutive days) in the Schengen on an US passport without any special visa, and these other countries usually let you stay as a tourist for 30 days without a visa, some 3 months.

Also Portugal has the Digital Nomad visa for 1 year renewable. That requires employment in the US (or elsewhere outside the EU) so not the same as what my brother was doing but a good option as the min wage requirements are not high; could get by with what would be part time work on a US salary esp in tech.

Given that op talks about not staying in the Schengen zone too long, I assume they just used tourist visas.

When I lived in Ireland I occasionally met people who would bounce between Ireland/UK and Schengen to reset their visa clocks (people from US, Canada, etc. can spend 3 months at a time, or 6 months a year, in Schengen, though eventually you might start getting tougher questions).