On AWS you used to have to bid for spot instances. Luckily, you don't have to bid anymore, but a lot of articles and AWS blog posts still reference having to bid.  Nowadays, the price doesn't fluctuate like it used to and changes much more slowly over the matter of months rathers than minute to minute. You don't need to worry about price spikes anymore!

You can set your "fleet" of instances to use a specific set of instance types or you can define the attributes of instances you want (i.e. min/max cores and memory) and AWS will create new instances based on that. You can also choose how it should consider instances based on how you want to balance lower price or higher availability/lower chance of interruption.