I am starting a new journey on a new dev company in my town... so far, I have been told its for a company that sell software for banks... so I expect to find a large codebase... Is there any strategy that you guys follow to assimilate the large codebases ?...

Have patience and learn the language specification.
Check all places, and don't expect to see fast progress.

Just one step at a time.

If it is possible and there is time.... change some stuff locally and see what happens ;)

I inherited an application that was nearly 20 years old. I dedicated probably 2 years of effort in the first year.

I'm now the project lead for the product. nobody gets anything in without my approval because it's the core of the company.

even now, I find things that surprise me.

take your time, digest it, always make decisions based on data, and always ask yourself, "will this change expose my customers data?"