What happens when ISP blocks all DA's. Do we need to use a VPN? Are DA's blocked in Tor blocked countries? How a Tor client gets bridge IP when DA's are blocked?
If Tor is blocked, you should use a bridge (typically one supporting the obfs4 transport). You can get a bridge in the settings of the Tor Browser, or from https://bridges.torproject.org/. If you cannot access that website, you can connect to it using a VPN or use an alternative bridge distribution method:
Another way to get bridges is to send an email to bridges(at)torproject.org. Leave the email subject empty and write "get transport obfs4" in the email's message body. Please note that you must send the email using an address from one of the following email providers: Riseup or Gmail.
Censors will often try to learn all the bridges, but the Tor Project attempts to make it difficult for them. When you request bridges on the website, it limits the number of bridges you can learn by your IP address (actually a block of IP addresses). If you wanted to learn many bridges, you would need to query the website from many different IP addresses in different locations. And when you request bridges through email, each email address can only learn a few bridges, so the censor needs to defeat Gmail's captcha/spam defenses and make many Gmail accounts if they want to learn all the bridges.
While this makes it difficult for censors, it's not impossible for them to learn many bridge relays. Because of this, some people run bridges that aren't published to BridgeDB (the bridge distributor powering the tools above). To get one of these private bridges, you need to learn about it from some other source (for example a friend running one in another country).