Questions that ask several other questions aren't the best format for Stack Exchange, but I'll answer these anyway.
But is there a sim card slot in a tablet if you want one?
Depends on the model. Some do,
but these are obviously used for just data and this can be used for internet access when outside of a Wi-Fi zone. Many mobile networks provide data-only SIM cards that are just used for internet which are perfect for tablets, but if you get a full SIM, with texting and calling, you'll be able to text but have a difficult time with making calls (I don't think most tablets even come with a dialer app?)
Do all tablets have WiFi so I can link to my home broadband router. (I found out today that phones do - that shows you how green I am.)
I think it's safe to say that all tablets aimed towards the average consumer have Wi-Fi built into them. ;^)
Just read the specs of the tablet before you buy it and you'll be fine.
Do they all come with a usb slot?
Again, it depends on the model, but all have some way to connect them to a charger. Micro-USB and USB Type-C ports are commonplace nowadays.
Can tablets and phones both do Google Play?
Yes assuming that the manufacturer installs it in the first place. The only time you'll not find it is on Amazon Fire devices, specialist devices, and in countries which prohibit Google Play due to laws or whatever. There are ways to install Google Play yourself though.