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.
For very senior staff they can get an injunction against you, stopping you from working elsewhere. For lower staff they can withold unpaid wages and charge you for the cost incurred of getting the temp cover up to the notice period. Given how high temp/contractor wages are compared to employee costs, this can be an insane number, especially in tech where an employee might be on £200 a day, but a contractor might charge £500-1000 or more.
The flip side is that other employers understand that you can't fill a position immediately. There's not many circumstances where a unique opportunity really appears on no notice.
In situations where you are in dispute with your employer and want to walk off, the traditional solution is to file a number of disputes and get yourself signed off as sick with "stress", then quietly negotiate a mutually agreeable exit.
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.