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.
PHB p. 187. "Downtime Activities"Activities," sub topic "Crafting"-topic "Crafting."(PHB, p. 187)
The general level of effort required is 5 gp per character day spent. Arcane focus from page 150 costs 5-20 gp, depending on it's form. Holy symbol typically 5 gp, same table. Druidic focus tops out at Yew wand, 10 gpitem creation.
Examples from Adventuring Equipment Table (PHB, p. 150):
Arcane Focus (varies); 5-20 gp, 1-4 character days depending on it's form.
Holy Symbol (typical); 5 gp, 1 character day.
Druidic Focus (varies); Yew wand, 10 gp, 2 character days.