If you have a worldview that includes gods, spirits, fairies, the universe as an entity, etc., that worldview often also provides you with the “meaning” bit. It can be stifling, reassuring, motivating, or depressing, depending. That was me for a few decades. Without that set of beliefs there is no built-in meaning afaik. You can study the stars or atoms or human behavior or plants your whole life and those things will not reveal a purpose or meaning for you, the universe, or humanity.

In the absence (for me) of any built-in meaning or purpose, we make our own meanings. If your meaning is “nothing matters so fuck it,” that is the meaning you are choosing or accepting as some kind of default. Like many other people I choose meanings around happiness: the greatest good for the greatest number, as Spock (and probably some lesser figure) said. In this mechanistic universe we somehow came to be, and we can think and feel and understand and learn. That is almost unimaginably amazing to me. We are people, not just idk viruses grinding away. I choose a set of meanings that value people and their happiness. Life is miraculous and apparently rare. In that special group we, humans, are the most phenomenal thing we know of in the universe. I choose to value us.