Up until Raditz first appeared in Z, Goku never even knew he wasn’t actually related to Grandpa Gohan. Since then we’ve seen Goku’s lineage grow and prosper. Below is a great family tree diagram I found online.
As you can see, Goku’s parents are Bardock and Gine both of whom are first generation Saiyans, 100% Saiyan blood. They have two kids, Raditz and Goku who are also both pure blooded Saiyans.
Before Planet Vegeta is destroyed, Goku’s sent off to Earth and here he’s adopted by Grandpa Gohan. He breeds with Chichi on earth who happens to be an earthling and the kids they have ( Gohan and Goten) are both human-Saiyan Hybrids, or third generation Saiyans.
Fourth generation Saiyans are seen too, namely when Gohan and Videl (who is 100% Human) have a child named Pan (Goku’s granddaughter), which makes her only 25% Saiyan by blood.
Sometimes it’s surreal when we are able to witness characters grow and evolve their entire lives into actual individuals with their extensive relationships and family dynamics, it’s part of what makes a long running show like Dragon Ball so compelling and relatable.
I also personally find it bittersweet watching your favorite characters grow old with you, watching Goku grow from the little Saiyan baby we first saw into the Grandfather he is now is always a tearjerker.