The big platforms let you officially download your own photos and videos. That is safer and more reliable than a separate downloader, and it is fully permitted.
Go to Settings → Accounts Centre → Your information and permissions → Download your information. Choose your account, which data (for example only photos and videos), the time range and the format. You get an email with a download link.
Go to Settings and privacy → Account → Download your data, choose the format and request the file. You save a single video of your own with Share → Save video, if that option is enabled for the video.
Your own videos: open YouTube Studio → Content, click the three dots on the video and choose Download. Everything at once (videos, playlists, comments): use Google Takeout and choose YouTube.
Go to Settings → Your account → Download an archive of your data. You have to confirm your identity and get a notification when the archive is ready.
Go to accounts.snapchat.com → My data, choose what you want (for example Memories) and request it. You get an email with a link.
Menus and names change regularly. If a step is not exactly like this, look in the settings for "Download your data" or open the help page above. An export often contains files you do not expect, such as messages and location data: keep the archive safe or delete what you do not need.
Want to decide the format and quality of a video that is yours or that you have permission to download? Use the yt-dlp command builder. Other people's videos belong to their makers; we do not help with downloading those.