Group-level/varying/'random' effects syntax in `brms` formula similar to `mgcv` vs `lme4`?

[jsocolar]

Some very useful resources for interpreting the Stan code:

Random effects and penalized splines are the same thing

Better priors (non-flat) for GAMs (brms)

I appreciate that this is now an older posting and that @martinmodrak has provided an answer that solved @DominiqueMakowski’s original question. But I thought I’d briefly comment on this aspect of the question In the toy example above, there is one fixed parameter (swt_1 ) which I assume is related to the smooth term (s(wt) ). However, I am not sure what does it mean (does it control the wigliness of the smooth term? or some other features of the splines?) in case others arrive here while lo…