As both flights are on one airline your bags would be tagged to next destination but as SFO is your 1st arrival point your clear immigration etc then you should only need to drop bags at the transfer area rather than having to go to a check in counter like you did when left you should get both boarding passes too when leaving

The US has a system, which I'd call somewhat unusual globally (like a lot of US systems), whereby you collect, touch, and hold your baggage after an international arrival and before a domestic transfer, but you do not (unless requested by staff) actually open it or do anything with it except walk with it a few meters from the baggage carousel to a domestic-transfer conveyer belt, all within the secure customs area, of course under the eyes of US Customs & Border Protection staff.

So that's what you will do. If it sounds confusing, rest assured that hundreds of other people will be doing it too, and it should be explained in an informational video on the plane. Your luggage will be tagged for Seattle already, so you don't need to do anything else in that regard.

After you put your bags on the transfer conveyor belt, you walk into the SFO terminal and find your way to the domestic United terminal. Everything we'll be well-signed, so just follow the arrows to your domestic gate number.

The TL:DR is:

"Will we need to collect our luggage and re check them in for the flight to Seattle?"

--Collect yes, check in no.

"Had hoped that our luggage would be checked all the way through as our flights are ticketed together."

-- It is checked all the way through. However, you will also be briefly in possession of it, serving as the conduit to get it on the domestic conveyor belt. I suppose the system's purpose is: travelers are (briefly) in physical possession of their luggage if CBP staff have any concerns about it.

After customs at SFO, be very careful to follow the connecting sign and not the arrival sign. They'll point to opposite directions. United should come into a G gate, connection exit is to the left, arrival exit is to the right. (Recently some United arrivals will use an A gate, and directions will be opposite.) After exiting into the public area (where you shouldn't see greeters). turn left and the United baggage handlers will be there to inspect the tags and put the bags onto the belt, then you walk further and find the security checkpoint.

Use the SFO interactive maps ( https://www.flysfo.com/maps ) to zoom in and out, change levels, and explore!

Thank you all for the helpful information, have answered my questions.

For future reference: It’s probably way too late to change your reservation, but if you wanted to check through your baggage to SEA without having to touch it at all during a layover, the correct approach would have been to fly Air Canada through YVR, rather than United through SFO. It’s unclear why you booked United, as Kayak is showing that Air Canada is cheaper on the SYD-SEA route.

Canada is a party to a US preclearance treaty, while Australia is not. Air Canada photographs US-bound checked baggage at check in. When you go through US immigration at a preclearance airport like YVR, the CBP officer shows you the photograph and watches your reaction as you are asked to confirm this is your baggage. So this saves passengers from having to collect and recheck baggage.

Air Canada is not well regarded.

Also, SFO isn't a huge mega airport like so many are around the world. Getting between both terminals is pretty easy and straight forward. It's not like CDG or LHR where it's easy to get lost!

coolcaesar we chose United as the price was very good(business class), friends tried to book the same flight two days later and the price was $7000 more for the two of them. Also Air Canada does not have a very good reputation. Also chose SFO for USA entry rather than LA as we thought it will be less stressful!!!!