Unless you've managed to get a switch made out of superconducting material and you're working inside a liquid helium bath, everything has resistance :)

You'll have a bad time trying to measure such low resistances, so usually you can look at the datasheet for the switch you're using or for a similar switch if you need a ballpark number. You'll see an on-resistance, and a max current (since there will be arcing when you open the switch).

If you do want to measure the low resistance, you'll need dedicated meter with a kelvin connection.

Every material has resistance.

You can calculate a materials resistance using its resistivity and dimensions. For a simple wire, the formula is R = p*l/A, where p is resistivity, l is length, and A is area (cross sectional. Imagine cutting the wire, you'd see a circular cross section).

Some materials like copper have very low resistance.

Some materials like oil have a very high resistance.

Some materials like carbon have a resistance somewhere in between, but generally fairly high.

Resistors use a strip of carbon to make a high resistance path.

Resistance in a switch is typically minimized by design, so introducing a switch or button should not introduce a lot of resistance. It is tyically better not to use switches of this type in signal critical or high power applications (e.g. sound or battery charging), but charging up a small capacitor or powering a small dc circuit should be fine

To measure resistance, one can use a multimeter. This makes use of Ohm's law. Ohms law shows that

V=I*R

Where V is voltage, R is resistance, and I is current. When a small voltage is applied across your component, the current is measured. Then using the current and voltage, it can figure out what the resistance is. It shows it to you on a display so all you have to do is touch the probe tips to the two legs of the switch.

For a switch this will typically be less than 1 ohm. If you buy from a reputable distributor (e.g. digikey, mouser, arrow, farnell, even LCSC) you can get the "datasheet" and look for the constact resistance. This might be a bit harder with ebay/amazon/aliexpress parts, so just stick with a multimeter.

A cheap handheld one is fine, but I'd say prob look for an EEVBlog or other video looking at good cheap meters; you can get pretty good stuff without breaking the bank. Don't stress over it though; any multimeter is better than no multimeter

Realistically, you need to put together a circuit and measure it with a Multimeter.

Some resistance? Yes.

Significant resistance? No.