My understanding is that base load tends to refer to a source that produces constantly to cover base usage levels round the clock. That’s what nuclear is good for, and then you have other technologies that service the peaks it can’t quickly scale to.

That’s not really a relevant model any more with renewables, what you need is in-fill for times the main power source isn’t producing well. As a complementary power source you want something agile and switchable. Usually this is gas generators which are easy to spin up/down more or less instantly. Obviously gas is not ideal as it’s still a fossil source, so some countries are looking at batteries etc to service those loads.