When Blocks and Sections are assigned IDs?

I've worked on maybe 8-10 Squarespace sites injecting custom css and code into previously published websites, but now I'm working on one that hasn't been published yet and it almost seems too...easy?

By that I mean I'm using actual class names, instead of something like this

.block-id123456789 {custom css}

I'm able to use actual class names like so:

.header-nav-list {custom css}

Is there a point in time when block IDs and section IDs are actually assigned? I'm glad I'm getting all of my code to work now, but I'm nervous that when the time comes for blocks and sections to have IDs, especially for custom JS and using querySelect(), that I'm going to have to put a lot of hours into redoing everything I've done so far.