Well, google, it depends.
I do a lot of work on low pressure (less than 15 psig)large diameter air lines, and it is common to simply drill and tap the pipe if it is schedule 40 or similar.
For small diameter pipes the tee approach is best.
For thin wall pipe at low pressure you need to drill the pipe and weld a coupling to it to mount the pressure.
For high pressure pipe you want to use a weld-o-let for the tap.
For high pressure rated pressure vessels you need a "nozzle" in the vessel wall.
For intermediate pressure pipe applications you can use a saddle tee from Grainger, McMaster Carr, etc.
You can mount the transmitter remotely and use a tubing connection for gas, but with a liquid you need to affset for the height difference between the tap and the transmitter.