How do people get their TikTok videos to look so crisp on Insta?

I've always wondered how so many just download and repost their TikTok videos on Insta, and it maintains the same crisp quality. When I download, the quality decreases significantly. Even when I edit on my phone, it's not enough to get the original quality back. So what's the secret?? I need to know because so much time would be saved if I just downloaded the right way.