You need to do more research.
UK, USA and AU embassies do not provide the letter you refer to.
Re money in bank method for extensions retirement it is 800k two months in bank prior to application for extension and 3 months post application then not below 400k and back up to 800k prior to next application for extension.
OR
Monthly deposit from overseas of 65k.
For extensions based on marriage requires 400k for 2 months prior to extention and in many cases for the consideration period after application.
After that can be withdrawn.
OR
Monthly income 40k per month.