You absolutely can prepare for system interviews it’s just some of the best content for doing so is behind paywalls. I opted to buy a course on leetcode covering this topic and my performance before was like night and day. Going through about 20 worked examples each of which has a 45 minute explanation means learning about 60 patterns you can slot into various problems. Once you’ve seen the patterns it’s a lot easier to apply them in different contexts to piece together the system you want.