Walmart is a massive logistics company that dwarfs any other - save maybe Amazon (and it would be interesting to see the break down side by side).
I believe it is dangerous to put the FAANG companies on such a pedestal.
Consider even McDonalds Tech Labs - which sold to IBM which was doing some rather interesting things with machine learning, and bilingual natural language processing (drive throughs).
My point is one of "Walmart to Google is likely much less of a jump than people imagine... other than the name attached."
The engineering practices at Walmart are quite similar to those at Google.
As someone who also made the transition from a smaller company to Big Tech, I can say that while the user base is much larger, the engineering practices and quality remain relatively consistent.
If you’re looking to land a job at a Big Tech company and want to practice by solving LeetCode problems, go for it. However, keep in mind that solving these problems alone won't guarantee you a job.