What do you call those recurring short videos at the beginning of each TV episode

The round item you have indicated is a radio button

radio button

noun

(computing)

​(on a computer screen) a small circle that you click on in order to make a particular choice. The radio button is then marked with a dot (= a small round mark) to show that it has been selected.

Radio button (Oxford Learner's Dictionaries)

The difference between checkboxes and radio buttons is that you can check more than one checkbox in a group of checkboxes, but select only one radio button in a group of radio buttons.

The name comes from old-fashioned radios and TVs with buttons (round or not) which could be tuned to different radio stations or TV channels. If one button was already pressed in, and you pressed a different one, the first button popped out, and the second one stayed pressed in.

Note: Although British English speakers are familiar with, and use, 'check', many call this symbol ✓ a 'tick', so we may say we 'tick' or 'untick' 'tick boxes'. Interestingly, this UK illustration for an article about 'tickboxes' shows a sheet headed 'checklist':

Another term that is commonly used for this situation would be a "filler episode." However, in this case, the term means that the episode is not considered important viewing for the overall plot. It's possible that a side character's motivation may not be considered filler if the viewer considers it to be important.

"Filler episode" also has a more negative connotation than "standalone episode." The former implies that the viewer could skip the episode if they choose, while the latter does not.