Marshmallow has a feature that lets you adopt the sd card as part of the internal storage. This requires encrypting the card (possibly reformatting - can't recall) but after that it works as an extension of the phone's own storage. This does mean that you lose some flexibility (you can't just swap the card) and means putting important data on a less reliable device (sd cards do fail), but it's an option.
That is, as long as the manufacturer has left you the option. Reports this week say that Samsung (S7) and LG (G5) have removed it from their software.
"Internal" and "external" are not options for mounting the sd card. "External" is the actual sd card. The "internal sd card" is a part of the internal storage emulating the sd card. On most devices this shares physical space with the /data partition (apps and app data) so you would gain nothing by moving apps there - I guess the sort of cheap Chinese devices that keep /data and /sdcard separate (the Gingerbread storage model still) won't have 6.0 yet.
You don't say whether you updated the phone or replaced it, but if you replaced it it may be relevant to note that moving apps to SD was deprecated several years ago. Some manufacturers put it back, but you've not been able to assume all phones will have it since 2011-2. So strictly the absence of a "move to sd" isn't a feature of 6.0 but of 4.0 and subsequent releases.