How to enable Database search & autocomplete live search?
You'll probably have to write a custom module for this. 

Here's the basic idea:


You should decide with which search mechanism you want this to work. For the sake of simplicity, I'll use the core Search module as an example.
Declare a new autocomplete field with your custom callback path; it might work if you just alter the search field type, turn it into autocomplete and add your custom callback path. I'm not sure if this will work in D8 though.
Whenever user changes the input, your module should query the database and see if there are any matching keywords stored in the search module. You can include any additional logic here, e.g. limit the search to nodes only, give some criteria higher visibility, exclude some keywords, etc.
Return back the similar keywords so user can choose some of them.
When search is submitted, display search results from core search module. If it's possible, the best would be to alter the search input field, so you don't have to much else to handle form submission, etc.


I didn't check much, but this should get you going. For more info on D8 form API, check this out. If you need to see how is this done in the core, check out the code for referencing the users (e.g. author field in node forms).

Hope this helps.