The privacyguides recommendations are usually a good start.
If you just want to transfer a larger file once, I've read about send.vis.ee positively here on the forum.
In theory, you could use any cloud storage if you encrypt first, for example via cryptomator. I'm not a friend of this approach since cryptography can be broken in the future and you never know what will still be stored of you by then.
The best option regarding privacy if you are about a secure file backup: Make 2 local encrypted backups on different brand SSDs or HDDs and store them in different and safe locations. It's much less convenient, but will keep your data offline and protected from theft or destruction. If you opt for SSDs, remember to plug them in every once in a while because they can lose your data if stored years without energy.