"Some parts are well established in the historical literature."

Most historians who have studied the topic accept those propositions because they are supported by abundant evidence.

It does not mean they are absolutely beyond dispute.

It means there is broad scholarly agreement.

For example,

"The Industrial Revolution required large numbers of workers."

This is well established because historians have examined

factory records,

census data,

employment records,

economic statistics,

contemporary reports.

So the statement rests on a large body of evidence.