For now it’s simple: all Let’s Encrypt certificates, either RSA or ECDSA, are signed by the same (RSA) intermediate certificate. Also, if you’re using the fullchain.pem file (if you’re using certbot as ACME client), which also contains the intermediate certificate, according to the nginx manual you don’t even need ssl_trusted_certificates.
For future references (if OCSP is still a thing) ssl_trusted_certificate is the CA certificate plus the intermediate certificate, not your fullchain.pem file (which is the leaf certificate + intermediate certificate)
In most Nginx releases, Nginx should be able to detect the root certificate for your intermediate (if the root certificate is in your CA trust store), which just means: you don’t need to specify this.
For dual stack certificates, especially certificates with different root CAs, don’t specify this value. Dual stack was supported in a later release and currently there’s no way (and no need) to specify two root certificate in the same file, if you have an up to date CA store.
If verification is disabled the server simply forwards to the client the OCSP response it received from the CA, without performing any validation.
Considering the network environment in Mainland China, I think it is necessary for me to enable it. But I also operate my website as an experimental project, which requires some research.