How to move the whole site with long URLs to WordPress?

I have hundreds, like 500+ on an old static HTML site.
It has a decent link and a logical structure that I do not want to mix.

Can I simply create posts (put the text and images there) and then simply "save as" the "old URL"?

Some of my URLs contain codes like 0542369abc.htm, which makes sense in my mind (in the organization of the articles, the structure, etc.).
And there are hundreds of interconnection URLs like this, that I just have to "move".

Another thing is that all my URLs end in page.php, but I see that WordPress usually cuts the extension like: page/
Yes, all my pages are .php in order to install more scripts.

I'm not sure if that is redirect or WordPress just "forgot" the extension or what …