taxes on foreign bank acc (personal savings)

the situation:

- no USA ties

- seishain income in japan

- owna no property or vehicle or business anywhere in the world

- cash savings (3 months emergency funds for one adult): some are in japan with yucho, and some in 1 foreign bank regular savings account

tldr: i know there are taxes to declare and pay. i don't know what amount is defined as taxable. also i want to know how much to set aside to prepare to pay for the next round of 確定申告. i always do 確定申告 online.

the questions:

1. the foreign bank account pays out an interest every month. this is subject to income tax. but how are these taxes calculated? and when i do 確定申告, what do i input and under what category/what box? (will i have to calculate it myself and declare it? if so, what am i supposed to calculate exactly?) for example will i need to fill in a specific form? will i need to submit bank statements?

2. the japanese yen / foreign currency volatility. let's say for example, yen keeps dropping in value. that means my savings in the foreign bank became more valuable in yen, even though all it did was sit in an account abroad and do nothing (it's not a time deposit, not any kind of investment) in this case is there anything taxable? if so, how is that calculated?

3. is any of this impacted/affected by when and/or how frequent i transfer my personal cash (from salary earned as a seishain in japan) to the foreign account?

my requests to comments

- please don't turn this into a debate about speculating the value of jpy

- please don't input my query into generative ai chatbots

TIA to anyone who knows where i can teach myself how to correctly do my taxes.