Mortgage lenders will use a standard mortgage application form developed by Fannie Mae that standardizes the loan application process. https://www.fanniemae.com/singlefamily/uniform-residential-loan-application
As a borrower, you will generally get it pre-filled with the mortgage information of the loan product that you have selected. Then it may take several rounds between you and the mortgage lender to completely fill out the application together accurate information. Refer back to the mortgage lender on anything that you don’t understand, because they are there to help you through the mortgage application process.