See for a short introduction to FPGA & Verilog: https://popolon.org/gblog3/?p=1478  (français: https://popolon.org/gblog3/?p=1478&lang=fr )

https://www.nand2tetris.org/ build a 16 bits computer and a tetris game, exclusively from NAND gates, from the Hebrew University of Jerusalem
https://github.com/wuhanstudio/nand2tetris-iverilog A reimplementation with iverilog (FOSS Icarus Verilog simulator in 350 lignes of Verilog)

https://github.com/wuhanstudio/picorv32_tang PicoRV32 on Sipeed Lichee Tang Nano

Headquarter of opensource FPGA synthesis https://yosyshq.net/ (based on Verilog-2005)

For VHDL: http://ghdl.free.fr/ with ghdl-yosys-plugin https://github.com/ghdl/ghdl-yosys-plugin/
: installation/usage: http://pepijndevos.nl/2019/08/15/open-source-formal-verification-in-vhdl.html