How to measure battery voltage
There is no one "usual" solution.  Some are:

Use a high side switch.  Use a fairly low impedance voltage divider, but only turn it on for a short time around each battery reading.  Since it's on for only a very small part of the total time, the average current draw will be low.  This is usually done with a P channel FET so as not to add a offset voltage.

Use as high a impedance divider as your A/D can tolerate.  A/Ds are specified for some maximum source voltage impedance for two reasons, to charge up the sample and hold cap within the specified acquisition time, and so that leakage current causes a small enough offset to ignore.  Too high impedance for the first reason can be overcome by a longer sampling time, which some A/Ds allow you to control.  However, there is nothing you can do about the leakage.  100 nA will cause 100 mV error with 1 M&Omega; source impedance, for example.

Use a high impedance divider followed by a buffer amp.  You still have to consider leakage, but good opamp input leakage is usually a lot less than microcontroller pin leakage.