1. Can I buy "America the Beautiful" pass?

yes, you can, but you can NOT buy the discount pass like senior pass. see https://www.nps.gov/fobo/planyourvisit/upload/pass-english.pdf

2. Does it cover all these places?

It covers all the national parks

3. Do I need to buy it online?

You can buy either online or at the park entrance if you arrive at regular business hours

4. Do I need to buy it at the entry of first national park?

Yes, the annual pass cover all the national parks for one year from the purchase day.

5. We are staying at the properties belonging to NPs at Grand Canyon, Zion, and Yosemite. Do we still need to buy entry pass?

The park entry pass is separate from your lodge. You need to buy the pass. But it usually covers the peak reservation, so you don't need to buy day-use reservation

"You need to buy the pass. But it usually covers the peak reservation, so you don't need to buy day-use reservation"

I may be misinterpreting the above statement. I'm pretty sure that you'll have to make a reservation simply to enter Yosemite in July, even if you already have an annual pass. I have a pass and had to make an additional reservation ($2) at https://www.nps.gov/yose/planyourvisit/reservations.htm for a visit this July, I also needed to make a reservation ($2) to get into Arches National Park last month.

Thanks Enzian and John P for your help and sending me these sites.

How do I know if my lodging already covers the entry fee? (I think I read somewhere that Yosemite lodging doesn't)

Folks sleeping inside Yosemite do not need a Peak Hours Entry Permit. See more in the Top Questions on the right side of the page ,full TripAdvisor website.

Thanks a lot PacificNWFamily and simba_8. I have found that for Yosemite, I do not need to pay £2 for peak reservation. But I still need to have annual pass ($80) or pay $35 on entering Yosemite park.

What I do not understand is, if I do not take annual pass, would I still need to pay similar entry fee of $35 at Grand Canyon, Zion and Death Valley, or it will be free at GC and Zion since I am staying at GC and Zion? (Basically, are the entry rules/fees similar to Yosemite?)

You'll still have to pay the entry fee even if you are staying inside the parks. So if you are visiting multiple parks, the annual pass is the way to go.

The entry is similar. Pay for a week or show the annual pass. San Francisco has a maritime park that is valid entry with That annual pass.

Thanks a lot ThanksSocialSecurity and PacificNWFamily for your help. Okay, I understand now. I will get annual pass on entering GC then.

I was so confused since afternoon. Thanks to your help and advice, its all sorted now :)

Most national parks charge $35 for a week, if you visit more than 2 national parks, the annual pass is the way to go.