https://learncssgrid.com/ This is one of my favorite resources for learning CSS Grid. I feel like it contains the right amount of explanation, with the right amount of visual examples to go along with it. I had a very easy time grasping concepts with this resource.
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout As always, MDN has the most in depth explanation on the topic, and covers more complex concepts than most other resources do. MDN is a good resource when you want an in depth guide to the entirety of the topic.
https://css-tricks.com/snippets/css/complete-guide-grid/ This resource, like my first one, was also nice for the amount of visuals to go along with explanations on all the different properties and values that go along with CSS grid. I am more of a visual learner, so I find these graphics very helpful.
Overall, just like using flex, there are several great resources online to learn the concepts of grid. While it's a lot, with enough practice and understanding, you can master the skills required to implement CSS grid in your web design.