Learn Dutch: We lopen door de eindeloze woestijn totdat we water vinden. - We walk through the endless desert until we find water.

The suffix -loos (meaning 'without' or '-less') attaches directly to the noun. While the Dutch word for 'end' is sometimes just het eind, it is very commonly het einde. When forming this adjective, the full base einde is used, resulting in eindeloos (endless).

Why does the 's' from the suffix -loos change to a 'z' in eindeloze?

This is a standard Dutch spelling rule. The base adjective is eindeloos. Because it modifies the noun woestijn, it requires the attributive ending -e. When a single s becomes sandwiched between two vowels, it changes to a z to reflect a softer, voiced pronunciation. This happens with all -loos adjectives when they take an -e ending (for example, hopeloos becomes hopeloze).

Could I use the shorter word tot instead of totdat to mean 'until' here?

In strict grammar, no, although you will sometimes hear native speakers do it in casual speech. Tot is a preposition used before noun phrases (like tot het einde). Because we water vinden is a full clause containing its own subject and verb, you must use the conjunction totdat to properly connect it to the main sentence.