In this answer, @bgoodri taught me that the scale parameter in rstanarm::stan_lmer, to which we assign a Gamma prior with the help of prior_covariance = decov(......), corresponds to the between standard deviation in the case of a one-way ANOVA model with a random factor (y ~ (1|factor)).

But what is this parameter for a general Gaussian linear mixed model? For example a two-way ANOVA (with/without interaction). And how to control the prior distribution on the group-levels variance components if there are multiple ones?