core-like, buried 4f orbitals
Why do all the lanthanides act so much alike, glow with such crisp colors, and refuse to use their f electrons in bonding? The single answer behind all of it is where the 4f orbitals sit. Picture the atom as a building: the 4f electrons live not on the roof where the chemistry happens but in a sealed inner room, screened by the 5s and 5p electrons that lie outside them. They are valence electrons by energy but core electrons by position.
Concretely, the 4f orbitals are radially contracted — they pull in close to the nucleus and tuck inside the already-filled 5s and 5p shells. So when a lanthanide ion meets ligands, those ligands feel and touch the outer 5s, 5p, and (in bonding) the 6s and 5d electrons, but they barely reach the 4f electrons at all. The 4f electrons are along for the ride, shielded from the outside world. Three big consequences follow at once. First, the 4f electrons hardly take part in bonding, so changing how many 4f electrons there are (which is the only thing that changes across the series) barely changes the chemistry — hence the near-identical behavior and the dominant +3 state. Second, because the 4f electrons are insulated from the surrounding ligands, transitions between f orbitals are not smeared out by their environment, giving sharp, line-like f-f spectra and clean luminescence rather than the broad bands of transition metals. Third, unpaired 4f electrons sit deep inside and behave almost like free-ion electrons, producing strong, characteristic paramagnetism.
This is the conceptual heart of the whole field, and the cleanest contrast with the d-block. The d orbitals of transition metals are exposed on the outside and so are strongly shaped by ligands — that is why crystal field and ligand field theory exist and why d-d colors shift with the ligand. The 4f orbitals are hidden, so ligand-field effects on them are tiny (a small fraction of those in the d-block). The honest caveat: 'core-like' is a very good approximation, not an absolute — there is a little 4f involvement in real bonding, and it grows for cerium at the start of the series; but for understanding lanthanide chemistry, treating the 4f shell as buried and chemically passive gets you almost everything right.
Compare two pink solutions. Cobalt(II), a d-block ion with exposed d orbitals, gives a broad absorption band that shifts and changes color the moment you add ammonia. Erbium(III), with buried 4f orbitals, gives a set of needle-thin pink absorption lines that stay in essentially the same place whatever you dissolve it in.
Exposed d orbitals shift with ligands; buried 4f lines barely move.
Calling 4f orbitals 'core-like' is a model, not the literal truth: they are valence-energy orbitals that happen to be radially buried. A small amount of covalency and 4f participation is real and is exactly what distinguishes some early lanthanide and actinide chemistry.