“then why do people nowadays say that Hashem provides us double parnassah on Thursday to account for Friday and Shabbat?”
Which people?
This was something unique to the Mann- double portion of mann was only on Fri- check out the chumash, when Bnei Yisroel came to Moshe on Fri puzzled why their portions measured twice as much as usual.
In terms of parnassa, nowadays we say that Hashem sets our parnassa for the week, if someone was to work on shabbos chas v’shalom, he would not be increasing his earnings. Rather, keeping shabbos keeps us, and brings us bracha in our endeavors.
I’ve seen this in my life. When I was in school, I was quite studious and found it hard to set aside studying on shabbos when I had tons of tests during the coming week. Since we were told that we were allowed to learn school subjects if they were chumash, etc, I would study for tests on those subjects on shabbos. Not technically being mechallel shabbos, but relying on a loop-hole. When I got to college and only had secular subjects, I no longer had the loop-hole and stopped studying at all on Shabbos, and shabbos was wonderful. And I saw the bracha- my grades were even higher than they were in HS for less effort.