Numbers work a bit differently in Japanese compared to English. Regular Arabic numerals are used in most cases, but there is also a way to write numbers using only kanji. This is useful to learn as a first step because it helps explain why (larger) numbers are pronounced the way they are.
Zero is usually written as ゼロ, but it also has a kanji version 零 and can be colloquially referred to as まる (circle). With Japanese numerals, zero is only used to represent exactly zero. This is unlike Arabic numerals, where zero is used extensively as a sort of placeholder in larger numbers.
There are characters for numbers 1-9:
There are then characters for powers of 10:
In Japanese, numbers are grouped into to ten thousands (4 digits) rather than thousands (3 digits). Arabic numerals are still written with commas every three digits in Japan (as everywhere else), which can make things confusing if you don't understand the system.
Each digit in a (non-trivial) Japanese number comes with a marker that represents its multiplier. There is no need for placeholder zeroes because each digit is marked with exactly how much it adds to the whole number.
For the Practice tab, it's more straightforward to start with learning how to convert Japanese numerals to hiragana before moving to converting Arabic numerals to hiragana.