i use Nara Baby to track what my baby does and it has a module for milestones. i also use pinterest and tik tok to see what other people are doing. my little man just turned one month and we do a lot of play time and tummy time and sometimes we just hang out with him cause he likes to be included
The wonder weeks app is really good and is just a one off £5.99
CDC's Milestones app is free
Pathways.org has been great! It even has videos to show you how to do the activities. :) and you can check off the milestones as your baby reaches them.