Hi I'm sure the manager would like to know your experience with signing in favour to being told they're wrong. There could be a few reasons why the mix of programmes happen along with the self-taught suggestion. Try not to let this rile you too much, find out how the different systems are used in your area - with parents on TV, through parenting classes, promoted through the local clinic and health visitor, and what training exists so maybe the option to upskill is there.
Finding credible free resources online is a risk,
makaton babysign & BSL all have their own investments to ensure, ASL is potentially the most open source, youtube can be helpful if you've had training to check a video's reliability.
If it helps identify with colleagues signing the
Lets Sign resources from Cath Smith are good
Best wishes