The universe seems pretty infinite when viewed with our current tools and from our perspective. I would still argue that we can’t really be sure just yet. However, we can say it’s effectively infinite just like a lot of things in physics are effectively massless, effectively frictionless etc. You totally can make your calculations work really well even though your model cuts some corners here and there.

In many cases, you can even assume the Earth is flat and simple maths still works well enough. However, when you zoom out and start doing more complex calculations, you run into trouble and need to upgrade to a more sophisticated model. I would argue that the current assumption of the universe being infinite can fall into the same category.