This is my understanding without doing any research on Lemmy specifically and what, if any, differences there is compared to others.

does admin has access all of our data?

Always assume that the answer is Yes. Regardless what you are using within the fediverse.

can he sell the data to some big corporation?

Again, Yes. Buti think they would be breaking their own privacy policy:

Looking at https://lemmy.world/legal and it refers to https://mastodon.world/about where general rules are clearly laid out that is also enforced afaik on lemmy.world. Them linking that page makes me assume that the same privacy policy that is used on mastodon.world also applies here, this can be seen at https://mastodon.world/privacy-policy

Do we disclose any information to outside parties?

We do not sell, trade, or otherwise transfer to outside parties your personally identifiable information. This does not include trusted third parties who assist us in operating our site, conducting our business, or servicing you, so long as those parties agree to keep this information confidential. We may also release your information when we believe release is appropriate to comply with the law, enforce our site policies, or protect ours or others rights, property, or safety.

Your public content may be downloaded by other servers in the network. Your public and followers-only posts are delivered to the servers where your followers reside, and direct messages are delivered to the servers of the recipients, in so far as those followers or recipients reside on a different server than this.

To be clear, the admins do not need to sell the information you share publicly, afaik it’s already freely and available in the open to anyone (as mentioned above in the privacy policy) and there is nothing stopping any outside party from scraping this data.

They do need to update https://lemmy.world/legal though asap to make things clear.