Streaming services decide what content and quality they offer to which Widevine CDM level. Although Widevine or Google, really, recommends what quality to offer which Widevine CDM level, though not everyone adheres to these recommendations.
L1, like I said, is the 4K holy grail. It’s the highest Widevine CDM level and as such enjoys the highest level of access. L2 is irrelevant. L3 is a weird one, Google recommends 720p for L3 Widevine CDM from Android devices, but 1080p for ChromeCDM, which is an L3 Widevine CDM shipped with Google Chrome. For some godawful reason, select streaming services allow 4K content for L3 Widevine CDMs, which is why one streaming service can have more 4K WEBDLs than another streaming service despite an identical content roster. Most streaming services serve 1080p content to L3 Widevine CDMs.
How can you get an Widevine L1 CDM, you may ask? For one, you can’t buy one, that’s always a scam. Sometimes, there’re leaks, but streaming services revoke these leaked CDMs quickly. You can break the Qualcomm Trusted Execution Environment (TEE) to get an Widevine L1 CDM from Android devices. But the TEE is a high-profile target for everything Android related, so chances are it’s not gonna happen. But this is what they want you to do. Instead, ignore the TEE, hack manufacturers and issue your own Widevine L1 CDMs, or steal Widevine L1 CDMs from freshly produced smartphones, smart tvs, etc. Alternatively, try to work yourself into a position where you can steal Widevine L1 CDMs. Or operate a legal business and become a Widevine L1 CDM issuer yourself.
Widevine L3 CDMs are a solved problem. Dump them from rooted Android devices. ChromeCDM requires software reverse engineering skills.