Mundane items can be crafted as per the rules in the PHB, given time and materials, based on the items normal cost. It's up the the DM to decide if any specific tools and/or tool proficiencies are required to do so.
According to the descriptions, all of these focuses are used to channel magic, but are not magical items themselves.
"Downtime Activities," sub-topic "Crafting."(PHB, p. 187)
The general level of effort required is 5 gp per character day spent on item creation.
Rangers Don't Get Foci
All classes that use a focus have a "Spellcasting Focus" subheading to their Spellcasting (or Warlock) feature:
- Wizard (arcane focus)
- Warlock (arcane focus)
- Sorcerer (arcane focus)
- Paladin (holy symbol)
- Druid (druidic focus)
- Cleric (holy symbol)
- Bard (musical instrument)
The wizards feature for instance reads:
SPELLCASTING FOCUS
You can use an arcane focus (found in chapter 5) as a
spellcasting focus for your wizard spells.
PHB 114
With the exception of the Ranger and Eldritch Knight, which specifically don't mention foci as part of the spell casting feature. Who have no such text.
It Is Intentional
Jeremy Crawford clarified in a tweet that the omission of focus for the ranger class is intentional. He was asked:
[D]o rangers use spellcasting foci, and/or do they need to buy component pouches at 2nd level?
And replied:
The ranger doesn't have a spellcasting focus. The trusty component pouch will do the job.
https://twitter.com/JeremyECrawford/status/687417277231267844?ref_src=twsrc%5Etfw
This makes sense as most rangers would likely have a bow. All bows, even the hand cross bow (even with Crossbow Expert), requires a free hand to load. So holding a focus would mess with the traditional ranger with bow. You keep a free hand for spells, pouch and loading firing arrows.
Mechanically
Using a bow, you have the weapon in one hand and the other on string. After you fire, your hand is free again. When you cast you pull out the components and put them back, or they are consumed, and your hand is free again. Works great with a bow. However, the arcane and druidic focus require object iterations, and you either have to drop it or spend the book keeping at the start and end of each turn.
Yes, a holy symbol could be worn and keep the hands free, likely why they did that for paladins, but they chose to go a different direction with the ranger.
Thematically
Ranger forage for food and materials, they are self reliant. It isn't hard to see them restocking their component pouch as they move through the wild area.
What about the Mistletoe?
it might be impossible to find mistletoe in the desert when a material component is needed.
The PHB says this about the component pouch:
Component Pouch. A component pouch is a small, watertight leather belt pouch that has compartments to hold all the material components and other special items you need to cast your spells, except for those components that have a specific cost (as indicated in a spell's description).
PHB 151
The pouch has all the components your spells require, and Material Components aren't consumed unless the spell says they are:
If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell.
PHB 203
So, you have mistletoe once, and you have it forever -- unless your DM rules that lose it or it goes bad, etc. Then you have to buy more mistletoe or buy a new pouch. Such a thing, however, isn't in the rule.
Unearthed Arcana / Tasha's Cauldron of Everything
A recent released play test (UA), and page 57 of Tasha's Cauldron of Everything (optional rule) provides the following to the Ranger class:
Spellcasting Focus
2nd-level ranger feature (enhances Spellcasting)
You can use a druidic focus as a spellcasting focus for your ranger spells. See chapter 5, “Equipment,” of the Player’s Handbook for a list of things that count as druidic focuses.
Best Answer
I asked Mike Mearls:
Can the arcane/druidic focus staff double as a quarterstaff?
And received the following response:
yes