Hi!

It depends on the connections… E.g. from Vienna to the West usually the first class is in the rear end and second class at the front of the train. Due to construction works this might be different sometimes.
There are monitors in the bigger stations (Vienna, Salzburg, Feldkirch, ...) or paper information sheets in smaller stations on the platform showing the order of the train. If you are early enough there, you can check the information screens and then stay in the right sector for your train.
On the main line there are usually two trains together between Vienna and Salzburg, where they split it up - one continues to München (or stays in Salzburg/Innsbruck) and other continues to Western Austria (Feldkirch, Bregenz)
Yes, you can walk through the carriages.

You can also check the trains here: https://www.vagonweb.cz/razeni/?&lang=en they show exactly the configuration of each train (but due to construction works sometimes might not be 100% accurate)

Thank you very much for the helpful info!

May I know what trains can I take if I have bought an Austria Eurail pass?

Am planning to travel within the Austria cities, eg Vienna to Salzburg to Hallstat to Innsbruck. Am I able to board trains operated by EC, RE, RJ, IC, EXP?

Thanks in advance for advice!

Eurail passes can be used on all trains operated by its participating companies, For single country passes this is still the case so almost all the trains in Austria will be included. The rail planner would advise you if  a planned train is outside the network, but they are normally limited services operated by private companies.

Note the planner will also advise if you inadvertently choose a station outside Austria (often a border station with 2 identities (usually differentiated by the country in brackets ABC(AUS) or ABC(ITA), Obviously this means you choose the ABC(AUS) for your Austria pass.

Thanks alot for the info!