Re: How we can be sure that we have sufficient backups for our sharepoint on-premises 2013/2016

24 hours of data loss is very high. A simple scenario of using SQL backups on a more frequent basis would solve that. Or do you mean time to recover (RTO) is 24 hours? To get a three node farm online (FE + BE + SQL) can be done in a matter of just a few hours, especially if you already have a VM template ready to go. Heck, I've built a full highly available farm by hand, including AOAG SQL, in under 4 hours :-)

> or the problem we faced is within the sharepoint application server itself, then how we can benefit from the sql backups

You shouldn't be focused on an individual SharePoint server. They're disposable. Again, have a VM ready to go that you can quickly deploy as a replacement server. And make sure you have highly available farms (2 FEs, 2 BEs, AOAG SQL). Yes, you need to have the server patched to the same or greater level which means you must keep the templated VM up to date to a certain extent.

> then can we build a sharepoint application server from scratch >> link it to the available databases >> then install the latest CUs for sharepoint (or a CU which is higher than the latest CU we had) >> run the product configuration wizard?

Not quite. You build your SharePoint server, patch it appropriately, then run the Config Wizard to join it to the existing farm.

Again, having multiple SharePoint servers per role will keep you out of this scenario except in cases where the entire data center is a total loss. The only solution to that is a remote standby farm or a fully scripted farm build and obviously Content/Service App database backups.