Externally mounted downtube batteries are the standard location for batteries on modern eBikes. Most common on diamond style frames, this battery's location advantages include better handing because the bike will handle most naturally the closer the battery is to the cranks where the pedals are. This location can also be used on step through designs - allowing easier entry at the same time having ideal weight distribution.
There are tools online for you to be able to estimate the range of any given battery setup, Bosch’s range calculator - select dual or single, other factors to compare. Specialized’s range calculator - select the model and other factors