Database structure for Craft 3 to import from external source?
I think you have to know what you need and what you want to achieve.
You need to decide if you want to add elements or records because there is a huge difference between those two types of objects and how you need to import and implement them

Elements
Pros:


it does not take a lot of time to make them manageable for clients
they can use all Craft features (relation fields, lists, edit forms....)
they are really flexible


Cons:


you'll need a little bit of time to dig into the concept if you are totally new to Craft
they'll have a record in your elements table so for larger projects you'll have to pay attention at your performance


Records
Pros


easy to handle
it does not require much time to create/import records since they are standalone and have no requirements
Yii's activerecord object is really powerful and intuitive in searching records
they don't create a massive overhead in your db


Cons


If you are going to make your records editable to users/customers in the cp you'll need to create everything on your own (templates/forms....) because they have no field layout and it's a mess to redefine them as elements to use Craft features... I did it once, it was not funny
You are not able to use Craft relations by default and have to create custom relation tables if you are going to do fancy stuff


After you decided what to use, you have to decide how you would like to proceed. You can create a custom element type or use an existing one (entries, categories) and how complex it needs to be. But without more information from your site it's hard to give any further advice.

I suggest everyone to create a custom plugin for imports to handle them. Due to Crafts/Yii2's plugin management and services it's totally trivial to create a data import because everything is really easy to understand and to manage. You can create a custom controller to fetch the data you need, set the values of your elements/records and use their save functions. The rest happens nearly automatically