[source: PhD in drug design specializing in G-protein coupled receptor pharmacology]
Drugs like LSD form non-covalent bonds with receptors. In a macro-world analogy, think of your hand sticking to a syrup-covered fork vs. a covalent bond being your hand stuck to a super-glue covered fork. Different drugs have different levels of "stickiness" (called affinity) for a particular receptor, and LSD has pretty high affinity for its target receptor, 5-HT2a, but it isn't permanently attached. In fact, affinity is defined by relative association vs. dissociation rates of drug-receptor complex.
On a microscopic level, each molecule of LSD is falling in and out of the receptor binding site stochastically. This leaves open the possibility of another drug binding to the same site (called competitive inhibition) when LSD isn't occupying the site. If another drug has a higher affinity, it will occupy the receptor more of the time. When you zoom out and consider the entire set of receptors and drugs, the sum of these individual stochastic events these effects follow characteristic patterns described by the law of mass action.
https://derangedphysiology.com/main/cicm-primary-exam/requir... is a pretty good description
*I'm simplifying parts of this somewhat.