For this approach, the key is policy routing as described here.
For each routing mark you define a routing table which has a different WAN as a default route with highest priority = lowest value of distance parameter. You can combine it with tracking WAN availability as used in the WAN failover explained here.
Bear in mind that
- by destination IP address you cannot tell youtube from other Google services, and telling them from one another by TLS-host is possible but too late for choosing a WAN.
- many worldwide services use short-lived DNS answers to control load on individual servers, so you have to add the domain name (youtube.com) as an address to the address-list, which makes RouterOS dynamically create other items on the same list and keep them up to date by sending a new DNS query as the old answer expires. Needless to say that the clients must use Mikrotik as their DNS server to get the same responses