I searched for the term "contrary" in reference implementation in the search bar and only got a PDF. I was expecting both PDF and Word doc to show as search results as both had the same text and content. I am not sure if ASC supports search based on text/contents within the document. Can someone con...

Yes , those are called hydrated assets . Details here :https://experienceleague.adobe.com/en/docs/experience-manager-cloud-service/content/assets/content-hub/upload-brand-approved-assets#manage-assets-uploaded-using-content-hub 

If more than one keyword is used, AND is the default operator between the keywords, thus, it considers a search for ‘apple mango’ to be ‘apple AND mango’. For results having both terms use apple OR mangohttps://experienceleague.adobe.com/en/docs/experience-manager-cloud-service/content/assets/best-p...

@Rajashankar33: May be you can use asset  collection if that works for your use case which only keeps reference. Or make use of metadata - like tags or drop down values to indicate the usage of assets .

Few questions :Do you have AEM sites cloud license ?How many assets and what's total size of assets you want to migrateWhat is your source CMS ? 

 Publish authentication -  million users  -  For a customer with a million end user base , would AEM Cloud SAML OOTB login  handler be able to support and scale without performance issues ? Any NFR's references will be helpful or should we be looking at developing custom handler outside of AEM ...

looking for approaches where we don't need customisation - if anything exists at all ?

I am aware of this , but I am specifically looking for bulk activation as a workflow step when payload is a folder not direct repliction.

 Is it possible to activate all the assets and child assets (in subfolders) with single workflow  instance/request in 6.5 AMS? AEM Cloud has 'Publish content tree' but 6.5 doesn't. If I select 'Activation Model' and use a folder as payload , 1 workflow instance is started and  only folder gets activ...

Like I said above , this is technically feasible but depends on use case. For example, is the intent to keep content always in sync or content sync periodically. As I mentioned , Custom Replication is technically feasible but AMS doesn't support/recommend replication from lower to higher env and if ...