1. Privacy against advertisers 2. Privacy against security agencies.
From my perspective, first issue, is more important than second one. I think Apple does pretty decent job regarding first issue. MS holds second position. Google/FB hold the worst position.
So even though Apple/MS are closed source, I think our privacy will improve considerably just by installing an ad-blocker and sticking to Apple/MS products.
Second issue is more involved and you'd probably need to give up on all sort of mobile devices. On desktop you have to move to Linux/BSDs. No online storage (or at least move to self hosted solution). No hosted email(Or maybe just protonmail, ...). Life becomes tougher, if you decide to improve privacy against security agencies.
People tend to treat privacy as a binary issue. (either no privacy or full privacy). However, as said, majority of people experience much higher privacy just by dumping Google/FB and using an ad-blocker.