Front-end is HDLs — (System)Verilog, VHDL, etc. Implementation and formal will be Jasper & its ilk. Backend (physical, etc.) use fab-specific bespoke software from the majors (Cadence, NXP, MG, Synopsis, ...).

The front-end stuff could be done by one person; Verilator is a great example (although it's now "in house" to NXP). Implementation, LEC, etc. are mathematically intimidating -- they're proof engines -- but doable by a small team.

Physical requires inside knowledge of the fabs. The fabs aren't going to let you participate unless you're a major, because it costs them a lot of money, and each additional participant is another potential leak of their critical IP.

The tooling is all "vertical" and starts on the backend. If you can't do backend, you're not a player.