In my startups, what I'm looking for is primarily three things: that the applicant's interests are in line with what the startup is doing, that the applicant will mesh well with the rest of the team, and that the applicant is good at problem-solving and working without a great deal of supervision.
The specific skills the applicant has are important, but probably not "top 3" important. As long as I'm comfortable that they enjoy learning new things and are good at it, any specific skill they lack can be provided through training.
When it comes to contents, "startups" come in all shapes and sizes, and they've got different requirements. There is no one-size fits all tbh.
1. Keywords present or CV goes to trash
2. Good intro (2 lines) at the top of CV. Good structure (don't list a "Skills" section just hanging around alone. Put the skills you have used next to the jobs in which you have used them). Don't say "Used Java and K8s and CI/CD to build microservices" but "Built a microservice to handle incoming email requests from a third-party company" (or something better). This is for Engineering Managers who read CVs (not all of them read CVs)
3. Good layout/fonts. If your CV looks like it has been generate with Latex or similar, that leaves a good impression. This is for Engineers who read CVs (not all of them read CVs)
So, in general make sure you know that your CV is going to pass through all the layers tech companies have nowadays (e.g., if your CV is the most strucutured and beautiful latex-generated CV in the world, but it doesn't include any of the keywords HR is expecting -> it goes to trash)
I know this sounds extremely cynical, but most(not all) startups are somebody’s grand efforts to make a shit ton of money for themselves (and smaller but meaningful sums for some people who join on that effort). There is nothing wrong here inherently just calling a spade a spade and if You are failing to see this reality you may not understand how resume is just a tool that allows you get to a starting point of convincing someone that they should go be you some amt of money in return for your experience, skills etc. they are going to give thst money to someone and there are many takers. So they will try to maximize their ROI by giving it to the person who is most promising for “their endeavor “. As such while no two startups are alike you can abstract out the common needs across them and optimize your resume (and your interactions) to that end.