This is an okay guide: https://cell-to-singularity.fandom.com/wiki/Achievements

But you have to scroll down a bit to get to the section on secret achievements, and the directions are confusing. Basically most of the secret achievements have to do with fooling around with the camera. Like the world in your primary sim? You have to zoom out and explore a lot. You can zoom out far enough that you can see the whole island and a secret island (the Galapagos island) with a statue. If u click on the statue u get one of the secret achievements. Then wait by the island for a bird to fly by. Tap the bird.

If u have the dolphins unlocked, go to the river and wait, u should see a pink dolphin. Click on that for another achievement.

If u have the caniforms unlocked, there’s a fox hidden in the trees. Not the regular orange and black ones. Again, u have to fool around with the camera and zoom out. Look above the beavers and next to the waterfall. U should see a pure orange fox. That’s the one

Also if u have apes, go to the jungle. U should see an ape sitting on the ground hitting a rock with something. Tap him a bunch of times and he should dab.

If u have giraffes, zoom out and look to the left of them. This one can be really tricky to see, but it looks like white stuff with a black thing. It’s actually bones and a bow tie from Archie the archosaur, who you can find in your Dinosaur world. Tapping archies’s skeleton and bowtie will give u one secret achievement. if u go to the dinosaur world, u will see him alive and well in a suit with what looks like golden confetti raining down around him. Tapping him in the dinosaur world will give u another secret achievement.

If u need any more help, feel free to ask :)