The short version: saving a public reel to watch later is almost certainly fine where you live. Posting it on your own account is almost certainly not. Nearly every real question sits in the gap between those two, so it is worth being precise about where the line actually falls.
Personal use
You see a clip, you save it, you watch it again next week. Nobody has distributed anything, nobody has lost a sale, and the creator's reach is unchanged. Copyright law in most countries treats private copies of this kind leniently: some jurisdictions have an explicit exception, others reach a similar place through fair use or fair dealing.
The condition is that it stays private. The moment the file leaves your device you are in a different category, and it is a much less forgiving one.
Reposting
Downloading a reel and uploading it to your own account is copyright infringement. Not a grey area, not a technicality; it is the textbook case. You have copied a protected work and republished it without a licence.
Two objections come up constantly, and neither works.
"I credited them." Credit is a courtesy with no legal weight. "cr: @username" in the caption does not create a licence, and it has never succeeded as a defence. It is polite, and that is all it is.
"They can just report it." They can, and Instagram's copyright process is genuinely effective: repeat strikes remove accounts. But the platform process is separate from the legal position. Not being sued is not the same as being in the right.
Music makes it worse
This one catches people who are otherwise careful. When you use a song from Instagram's music library, you are relying on Meta's licensing agreements with rights holders. Those agreements cover use inside Instagram. That is the whole scope.
Extract that audio and put it under a YouTube video and you are outside the agreement entirely: you now need your own licence, which you do not have. YouTube's Content ID will find it, usually within hours, and the outcome ranges from the revenue going to the rights holder to a strike on your channel.
The extraction itself is not the problem. keep the sound without the footage works fine and there are plenty of legitimate reasons to want an MP3. What you publish it under is where the exposure lives.
The genuinely unclear middle
Some uses are arguable, and anyone who tells you otherwise is overselling their certainty.
Commentary and criticism. This is exactly what fair use exists for. Reacting to a clip, analysing it, using a few seconds to make a point: a real critique that adds something is a strong case. A full clip with your face in the corner and "so true 😂" is a weak one. The strength varies with how much you used and how much you added.
Education. Most jurisdictions have a carve-out for teaching, with real conditions attached about scope and audience.
News reporting. Similar, similarly conditional.
What all of these share is that they are defences, not permissions. They are arguments you make after someone objects, which is a worse position than not needing one.
Where you are matters
Private copying is treated quite differently around the world. Several European countries have an explicit exception funded by a levy on blank media and storage. The UK is an odd case: its private copying exception was struck down by the High Court in 2015 and never replaced, so strictly speaking ripping a CD you own is still unlawful there, a rule nobody enforces and nobody has fixed. The US has no explicit exception but a broad fair use doctrine that usually arrives somewhere similar by a different route.
If money is involved in what you are doing, this is the point to ask someone qualified in your own country rather than reading further.
Instagram's rules are a separate thing
Meta's terms prohibit accessing the service by automated means without written permission. That is a contract between you and Meta, not a law. Breaking it does not put you in front of a judge; it puts you at risk of losing your account.
In practice, saving public posts you could already see draws no attention whatsoever. Automated collection at volume does.
The shortcut that always works
Ask. A short message saying you would like to share their clip with credit gets a yes far more often than people expect, because creators generally want the reach. One minute of effort removes the entire question.
Failing that, link to the original instead of re-uploading it. The creator keeps their view count, you keep the content in your feed, and there is nothing to argue about. Instagram's embed and share tools exist precisely for this and nobody uses them.