what's meaning of "expressly found that"

These is a legal expression, and you will not find it outside that context.

It is saying that the judge said specifically (rather than referring to it, or implying it by choice of words) that the risks were disclosed etc

the term find is the verb for the noun finding

It means:

Finding The result of the deliberations of a jury or a court. A decision upon a Question of Fact reached as the result of a judicial examination or investigation by a court, jury, referee, Coroner, etc. A recital of the facts as found. The word commonly applies to the result reached by a judge or jury. [bolding mine]

West's Encyclopedia of American Law, edition 2. Copyright 2008 The Gale Group, Inc. All rights reserved.

If a judge expressly finds [something] he or she is explicitly or clearly stating something.

Sometimes, in legal language, one can infer a judge meant this or that even if he or she did not expressly (clearly) state the thing.

The past tense is found.

You will often see in newspaper articles, expressions like: "The jury found for the plaintiff" (in AmE) and that means that claims made or brought by a plaintiff or respondent have been agreed to by the jury.