I made this exact career transition, the first ~10 years of my career were doing backend development at web companies (mostly Python, but later Go) and I switch

I made this exact career transition, the first ~10 years of my career were doing backend development at web companies (mostly Python, but later Go) and I switched to working in C++ in the autonomous vehicles industry. The key is really just to apply to these jobs, even if you don't have direct work experience. I had a blog on my website that had a lot of C/C++ stuff which I think helped a bit, but in the end I just applied to a bunch of jobs and eventually got a technical phone screen. I described some of the C/C++ projects I had worked on in the past (mostly personal projects). I got grilled fairly hard on C++ stuff in that interview because the company knew that I wasn't coming from a professional C++ background, but I knew my stuff so it ended up not being a problem. The other way to do this would be to try to get a job at a company like Google or Facebook. Typically you're just another person in their recruiting funnel and whether or not you know C++ doesn't actually affect how their hiring pipeline works. At both companies you're typically not applying for a spot on a specific team at the time of your interview, so you don't even necessarily have to write C++ in your interview to end up getting matched to a C++ team.