>E.g. You registered on Reddit and just used a random/disposable/freshly thought of email address like “[email protected]”. >Now if you receive an email from Reddit or anybody on the email I guess you can just hit reply and it will be replied from that Email

Yes, that's exactly how it works. I just went and checked it. I have one catchall alias set up with a name of King Charles on *@kingcharles.tld. Anything that comes into that domain like [email protected] I can reply to and the FROM will be King Charles <[email protected]> without me having to do anything. [It actually shows it as three textboxes "King Charles", "reddit", "kingcharles.tld" you can edit]

I pay $5 a month which I consider a bargain.