Learn English – “See you Monday” vs “See you on Monday”

They're mostly the same. "Based out of" often suggests that the subject maintains a headquarters or home office in the given location, but spends a majority or other significant amount of time working in other locations; "based in" suggests that the subject works in the given location most of the time. But counterexamples are common, so you shouldn't make assumptions based solely on the wording.

I am not a native speaker, but I believe that both I am seeking a position and I am looking for a position are correct, and I would definitely say and write with a company.

If you are not applying for an advertised position, you may write

I am looking for a position of Senior Developer.

or

I am looking for a Senior Developer position.

If you are applying for an advertised position then you would simply write

I am applying for the position of Senior Developer.