What kind of fitting do I need (under sink) ?

I am looking at adding a 1/2" tee to the cold water line under my sink. This will allow me to run my RO/DI water filtration unit.

Here is a link to an image of the tee valve used on this unit:

The soft tubing from the faucet is connected to the copper/brass line coming out of the floor with a 3/8 compression fitting.

I have disconnected that and replaced the 3/8" compression fitting with a 1/2" compression fitting. I then got a 1/2" nipple and connected the compression fitting to the female part of the plastic tee (nipple screws in tee, compression fitting screws on other end of nipple).

The trouble comes when I need to connect the bottom of the tee to the receiving end of the pipe coming out of the floor. The bottom end is 1/2" male and the line coming out of the floor is 3/8" male (or, at least a 3/8" compression fitting screws onto it).

What part/s do I need to connect these?

I’m thinking I need a union or something that has compression fitting type nuts on both ends?

Please forgive me I am very new to plumbing.

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