As one would expect, YouTube has myriad resources for the Welsh learner, ranging from brief clickbait like "An American tries to speak Welsh" to full episodes of educational programmes and everything in between.
The first port of call, really before going any further, is to watch the Welsh Plus series which go into great detail about how to pronounce the letters of the Welsh alphabet, both individually and in common combinations. They also touch on the mutations. Welsh Plus's playlist can be found here.
When I started learning Welsh, and before I'd discovered the Say Something in Welsh website, I discovered a series of videos called "Learn Welsh with Will" in which Will Huw, an Englishman living in Wales, disseminates his own Welsh knowledge as he goes about learning it. I liked Will's videos as he comes across as non-judgemental and very amiable. You can see Will's playlists, which include more than just Learn Welsh With Will, here.
Another series of videos I happened upon are aimed at adult learners from the 1990s, Now You're Talking (conveniently mis-spelled as "Now Your Talking" by whomever uploaded them!). These are presented by the likeable Elin Rhys in a variety of very early 90s outfits! There are 36 full half-hour episodes covering a variety of topics and gradually building vocabulary. I enjoy these videos as they present a chance to hear simple conversations in Welsh and learn everyday vocabulary. My only criticisms are that the programmes are now quite dates, the language is in some places a little too old-fashioned, and the pace of the lessons is a little slow for my liking. There's also not a great focus on grammar, so you'll need other resources to accompany these. You can find all 36 videos here.
Much more recent, are Margaret Hall's wonderful videos. Margaret is a Welsh learner and as part of her learning process she's created a series of videos in which she explores various places in Wales. Margaret speaks very clearly and uses a great vocabulary for the upper-beginner or intermediate Welsh learner. I find I can understand a lot of what she says, but not everything. That's due to limitations in my Welsh, though, not any problems with her speaking. Margaret's channel is here.
Finally, the Easy Languages series have several videos in Welsh. These are largely conversations with Welsh speakers in towns and establishments around Wales, conducted entirely in Welsh. The videos are subtitled in both English and Welsh, which is both a blessing and a curse, as you can find yourself reading the English subtitles rather than understanding the spoken Welsh. The ease of understanding the spoken Welsh depends largely on the speaker. Some interviewees are fluent speakers, some only beginners, but the most encouraging thing I took from these videos is that, actually, Welsh is very fluid, Welsh speakers are very forgiving of mistakes and, in reality, things like mutations are not nearly as stuck-to as online guides may lead you to believe. You can find those videos here.
There are plenty more besides, but these are the video series that I've found most useful and most enjoyable while learning Welsh. Feel free to comment if you feel I've missed anything important!
Da bo am y tro!