The memory installed on a Raspberry Pi is directly soldered and is not intended to be replaced or extended. Looking on the web, while there are quite a few posts asking for a method to add RAM, all that I could see indicate its not reasonably feasible. I'm sure it could be done with specialized equipment and components however the outcomes of the process of desoldering and soldering new parts would seem quite chancy for a functional Pi after doing so.
There are kits to add a Solid State Drive to a Raspberry Pi. These kits all use the existing USB port so any data transfer is going to be governed by USB bandwidth and transfer speed which is significantly lower than SATA transfer speed and orders of magnitude slower than RAM transfer speeds.
See this raspberrypi.org forum posting, Using your Pi with an SSD (Sold State Derive) which mentions both the X850 mSATA storage expansion board as well as building a solution from parts.
This Tom's Hardware article, Raspberry Pi 4 With an SSD: Dramatic Speed Improvements, Higher Price, describes testing such an SSD kit with both Raspberry Pi 4 and Raspberry Pi 3 and comparing it with using a flash drive.
The article says:
Chromium browser, the open-source version of Chrome, is probably the
most important preloaded app and the one you’ll open the most. It
loads a full 38% faster with an SSD than with the microSD card on the
Pi 4. Even on the Pi 3 B+, which is limited to a USB 2.0 connection,
the SSD was a full 24% quicker. Note also that, because of the faster
microSD reader, the Pi 4 opened the app 30% quicker than the 3 B+,
with both of them using the same memory card.
The real loser here and elsewhere is the USB Flash drive, which is
half the speed of the microSD card and a third of the speed of the SSD
on the Pi 4. Chromium was the only app we tested with the USB Flash
drive on the 3 B+, and the numbers show why. It’s painfully slow to
use a USB Flash drive, or at least the Patriot Supersonic Rage drive
that we used.