Yep. Instead of taking a class feat, you can instead take an archetype feat. So at level 2 you're able to take the Red Mantis dedication, and at level 4 you can then decide to take Basic Red Mantis Magic if you wanted. Or you could skip that and at level 6 take Crimson Shroud. Or if you do take Basic Red Mantis Magic, at level 8 you could spend your class feat on taking Mantis Form.
It's also how multiclassing now works in PF2, so if you're a Fighter and want to multiclass into Wizard, you'd simply take the Wizard Dedication feat in place of a class feat when you're at least level 2 and then take as many or as little of the available Wizard archetype feats as you want. You're basically trading away the things that make you a Fighter in exchange for he things that make you more of a Wizard, creating a hybrid class.