Another "America vs Europe" (well, UK): I have my notice periods written in my contracts, whether that's one month or three months, and it is always bi-directional.

Giving up to 3 months of notice to change jobs is wild. What are the consequences if you tell them you're leaving in 2 weeks instead of the notice period?

Typically in US tech companies layoffs don't give employees a notice period, but they do give severance pay. So you stop working effective immediately but you either get a large check or continue to receive paychecks for a period. That period depends on the company but it's usually within the range of your notice period. You don't have to work during it, though.