I always preach "follow the exam guide".
There's a PDF you can download from certification.salesforce.com for every certification. You'll find it linked as "Exam Guide" under the Quick Facts section. That document shows you exactly what you need to know to pass the test, and how each subject area is weighted.
My preferred preparation method is simply to extract that outline from the PDF (copy and paste) into a text editor and go through it line-by-line, annotating each line item with links to the Apex Developer Guide or Salesforce documentation, or Trailhead. I color-code section headers by how confident I feel in my knowledge, and I always keep in mind the passing score, which for Platform Developer I is 65% - meaning that if you feel very confident about (well over) 65% worth by weight of the material on the exam guide, you can have confidence about your performance on the exam.
Salesforce is also now providing Trailmixes for many of the certifications, including this one (it's also linked from the page above). I find Trailmixes a little broad-but-shallow, personally, but I think they are a great start on an area that you're not familiar with that can then be deepened in the developer guides.
There are lots of external resources you can pay for that are more instructional in nature, and if that's your best learning style, you should take a look. But while these are not easy exams (there's a reason the passing score is 65% - guessing isn't a good strategy in many cases), Salesforce wants to set you up to succeed by telling you exactly what to know. Try that resource first.