While I myself can't give a comprehensive answer, I've had the pleasure of reading this article which gives a lot of good info on how old battlefield scrap and ordnance/ships/armor/small arms, etc., was processed in the various theatres after peace was declared.

The author of the blog is a little light on citing sources, but a lot of what he says is easily cross-checked enough that I feel it's a good entry-level dive into the topic.

The rest of the blog's posts are pretty interesting too, some are annotated and cited better than others. Very cool collection of topics.

In the Middle East, yes, this stuff was often just left to rot away. Significant swathes of the Egyptian and Libyan deserts are WWII minefields that it hasn’t been worthwhile to clear.

The continued danger and viability of these weapons can be demonstrated by the fact that ISIS is reportedly salvaging them to make IEDs: https://www.newsweek.com/2016/08/19/isis-hitler-islamic-state-egypt-world-war-ii-land-mines-jihad-bombs-explosives-488927.html