Variable Fonts: Do variable fonts interpolate between instances or some other methods?

Do variable fonts interpolate between instances or some other methods? How variable fonts work could effect the set up in order to have the proper accuracy or effect file size or other considerations like accuracy. if a glyph is continuous from largest to smallest weight, is it better to include just the end point instances, or should all the intermediary instances be included? When using braces or brackets, the inflection point of the change may rest at a different location than any of the typical instances; so, for the most accuracy should a new instance be included at the brace/bracket inflection point (because the variable font will interpolate between the instances), or will a variable font know the location of the separate inflection point used by the brace/bracket without including an instance. If a variable font interpolates between instances, then in the case of braces/brackets it is possible to only include instances at the largest weight, smallest weight and inflection point; and eliminate all instances in-between, or is it better to include all instances? Or the glyph largest and smallest weights are included in the masters and any inflection points are included in brace/bracket declarations, so is it that actually no instances at all are required? If this is the case, then why include any instances in a setup for variable fonts. Some understanding of how variable fonts interpolate would be helpful - Thanks.

A variable font can do without instances. But an app using the font may expect them, or only allow access to predefined instances. But instances are just coordinate positions in the design space now. Do not mistake them for masters.

There are lots of resources for deeper understanding Variable Fonts. Please read the Variable Fonts tutorial and also the linked articles, especially the first one by John Hudson.