VMWare ESXi is a bare-metal hypervisor. What this means in practice is that you install the software on an Intel based PC - usually of a server type specification such as a Xeon Processor. The software is NOT designed to co-exist with anything else on that PC. Thus you are unlikely to be able to dual boot ESXi and Ubuntu on the same PC.
The software does not have a GUI per-se and is not designed to be installed within another virtualisation solution such as virtualbox or Windows Hyper-V.
To create and install virtual machines you require another PC such as your laptop with VMWare vSphere client installed connected in a TCPIP local network to the ESXi hosted PC. There is no Linux variant of the full vSphere software. If you want to control ESXi via a Ubuntu desktop you need to control via a browser and VMWare Web Client.
ESXi is "free" but you need to obtain a license from VMWare themselves to keep running the software beyond the evaluation period.
The other type of software offered by VMWare is a type-2 (non-bare metal) virtualisation software called VMWare Workstation and VMWare Player.
There is no such thing as a "free" VMWare Workstation - you need a license and pay lots of money to VMWare.
If you want to use the "free" VMWare Player you should be aware that you can only use this in a non-commercial environment - i.e. home-use. For a business you need to buy a license from VMWare themselves.
The "free" virtualisation software VMWare Player is available from VMWare themselves. This software is installable on Ubuntu. Thus - if you are looking to install a VMWare virtualisation solution within Ubuntu - you should consider either VMWare Player or VMWare Workstation.
Both products are downloadable from the VMWare website.