Sideloading your own apps or open-source apps is legal. Downloading cracked paid apps is illegal. The act of sideloading is not illegal in most countries, but it violates Apple’s ToS (though they rarely enforce for individuals).
: The original tool created by Saurik (the creator of Cydia) for sideloading IPAs. While legendary, it often requires an Apple Developer account to work reliably on modern iOS versions. AltStore / SideStore cydia download ipa
Once you have a jailbroken device with Cydia, managing IPAs becomes much more powerful. Here are the essential tweaks for IPA handling: Sideloading your own apps or open-source apps is legal