How are Youtube guitar commentators syncing their videos?

There's no shortage of good guitar players on Youtube these days that talk technique, gear, etc. I assume they use smart phones for video, but the audio is usually professionally recorded. How does one actually do this, i.e., use a smart phone in conjunction with mics, preamps, and a DAW to produce video with proper audio?

Start recording on your DAW and on your phone. Then, clap. After you finish recording, sync up the claps. It's exactly the same thing they do in the movies, except they use something called a "Clapboard".

Sorry, I just realized I probably left out too much. I'm assuming you're making a video in REAPER. If so, you obviously have to upload the video from your phone into REAPER. That actually applies to any video program you might be using. Your REAPER recording and your video/phone recording need to be in the same program (whether that's REAPER or any other video software). You then move one or the other on the timeline until the "clap" lines up. You can then mute the audio in the video file by right clicking the item, selecting "Source Properties" and checking the box that says "Ignore Audio".

probably worth doing a clap at the end as well in case of any clock drift

Thanks for the helpful replies. I want to record video with iphone, audio with Reaper, and sync it in Adobe Premier Pro. I think I understand the process now.

Digital sample rate clocks are accurate and forgiving enough that program even up to an hour long can run without being perfectly accurately sync'd together before it starts to drift. Short song length pieces are essentially free. It's only for hours long stuff that you have to veri-speed one to match the other. So yeah, just line up a single clap and the whole thing is done and no one is the wiser. Everyone is taking advantage of that and just using multiple devices with abandon nowadays. :)

That's brilliant I never thought about why the clapboard was used, other than to mark a spot in video. Never occurred to me it would be to align disparate sources. Now I want one.

I guess I have to make a clapboard now so I can feign that I'm a pro.

I have a clapboard but ended up using my hands more because... 1. My hands are connected to my body and I never have to look for them. :D 2. Most of the time I used it was for recordings that weren't that loud such as acoustic guitar/vocal. The clapboard I have (like most I'd imagine) has a magnet/metal for the clap which gives the clap an extremely sharp and loud transient which clips the audio. I can do it softer but it wants to close loudly and that was half the fun. Nevertheless, I'm looking at it on a shelf right now collecting dust for two years. Mine has the dry erase marker etc. so I suppose that would be valuable if I needed to use that, but I never do. YMMV.

I have an iphone, and I separate vocal mic on a stand (usually just out of shot) and a mic on my guitar amp, both through my interface recording into Reaper. I clap my hands. Find the peaks in the audio, zoom in, and drag to line up the peak in the video camera audio track with the two mic tracks. I add a little compression so my quiet voice doesn't get too quiet, I keep all the loudest parts of any track below -10db LUFS-S and adjust the master output for about -16 LUFS-I. BUT don't spend forever thinking that top notch audio and lighting will make a channel grow or be popular. Content is STILL king. Look at that Scottish guitarist, Paul Stafford Cook, he does everything with just a phone. He has 20k subs. Why? Because people enjoy what he does, what he plays, and what he says. Top notch audio quality is NOT the important part. And it sounds just fine.