I think it was confusing because it was really an answer to two different questions.
Not all lenses can be used with just any camera body. So the answer to the question in the subject line is 'no'.
But the specific question was if the EF 135mm lens would fit a 90D body, and in that case the answer is 'yes'.
The letters EF stand for 'electro focus'. So all EF & EF-S lenses have electric focusing motors built-in. The EF are the original EF auto-focusing lenses and these work with any Canon EOS film camera or any EOS camera DSLR camera. There are a few lenses that don't have auto-focusing motors such as the MP-E 65mm extreme macro-focus lens as well as all of the TS-E (tilt-shift) lenses.
The EOS DSLR bodies with APS-C sensors have their own EF-S line (the S stands for "Short" back-focus because these lenses have a rear element that sits closer to the sensor than the EF lenses. Canon can get away with this on the APS-C sensor bodies because they have a smaller articulating reflex mirror that doesn't need as much clearance. So you CAN use an EF lens on these APS-C DSLR camera bodies, but not the other way around (you cannot use an EF-S lens on a Canon body with any sensor larger than APS-C susch as the APS-H or full-frame bodies).
Complicating things... Canon now has mirrorless lines. They now make lenses dedicated to these mirrorless bodies and these lenses have much shorter back-focus (shorter even than the EF-S series). BUT... Canon makes adapters so these mirrorless bodies can use the EF & EF-S lenses as long as you have the adapter (the adapter does not come with the camera, it's an accessory).
That sounds like a lot to remember... but for the 90D (a Canon EOS DSLR that has an APS-C size sensor) you can use:
(a) any EOS "EF" series lens
(b) any EOS "EF-S" series lens
(c) any EOS "TS-E" series (tilt-shift) lens, or
(d) the EOS "MP-E" extreme macro specialty lens
You CANNOT use
(e) the Canon EOS "EF-M" lenses, or
(f) the Canon EOS "RF" lenses
Those last two categories are designed for Canon "mirrorless" camera bodies.
"... the canon ef 135mm f2 lens is an amazing lens but will it fit ..."
Somewhat confusing answers but the answer to your specific question is, yes that lens will fit.
12/05/2023: New firmware updates are available.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.