Learn Russian: Физический труд улучшает иммунитет, притом это приятный процесс. - Physical labor improves immunity, besides, it is a pleasant process.

Притом (besides, moreover) adds a significant, reinforcing piece of information to the first clause. While и just connects two facts, притом emphasizes that the second fact is a bonus point—in this case, that physical labor isn't just good for your immunity, but it is additionally enjoyable.

Could we use причём here instead of притом?

Yes, in this context they are almost completely interchangeable. Both are conjunctions that attach extra information. Причём often emphasizes an accompanying detail, while притом leans slightly towards an added argument or reason (moreover, besides). Here, replacing it with причём would sound perfectly natural and mean the same thing.

Why use труд instead of работа for "labor" or "work"?

Работа is the general word for a job, specific tasks, or your workplace. Труд refers more abstractly to human effort, toil, or labor as a broad concept. The phrase физический труд is the standard set expression for "manual labor" or physical exertion.

Since труд (labor) is a masculine noun, why does the second clause use это instead of он?