Have a look at Chris’ video.
There’s some discussion in the daily photo thread as well here: The Daily Photo Thread - #4516 by DeanoG60
If you want one of those signature style ones, a mega lazy and free option is https://myphotosign.com/ which will send you a PNG.
For the stuff I post on social media I import into Lightroom Mobile, crop as needed and then in the export settings you can configure the watermark, which is saved to my files folder on my phone and just tell it to use that.