A website can open links in the same tab or use the target attribute to open another. In 2014 I asked why so many sites automatically opened external links in new windows. Often the owner feared losing the visitor. That is not a strong reason to control the browser for them.
Respect expectations
People clicking a normal link generally expect the new page in the same tab. The back button then takes them home. An unexpected window changes that route. Someone may think the back button is broken or struggle to find the original tab. The W3C advises opening new windows only when necessary.
The difference between internal and external links matters less than the visitor's purpose. A good outside source can make your article more useful. If readers want to return, their browser lets them. They can also choose for themselves to open a link in a new tab.
When an exception helps
My old article mentioned a form that someone has spent time completing. If a help link would erase their work, a separate tab could help. Better still, design the form so progress is not unexpectedly lost. A new tab can also make sense when audio or video should continue while someone reads extra information.
Tell readers what will happen. The W3C recommends a visible and accessible warning for links that open a new window. Then the behavior does not come as a surprise.
My rule is simple: let ordinary links do their ordinary work. Open an extra window only if it clearly supports the visitor's task, and make that departure from the default clear.